Annual report pursuant to Section 13 and 15(d)

Revenue from Contracts with Customers - Disaggregation of Revenue (Details)

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Revenue from Contracts with Customers - Disaggregation of Revenue (Details) - USD ($)
$ in Thousands
 3 Months Ended 12 Months Ended
Dec. 31, 2019
Sep. 30, 2019
Jun. 30, 2019
Mar. 31, 2019
Dec. 31, 2018
Sep. 30, 2018
Jun. 30, 2018
Mar. 31, 2018
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Disaggregation of Revenue [Line Items]                      
Revenues $ 306,612 $ 212,026 $ 198,183 $ 150,112 $ 217,371 $ 205,375 $ 196,982 $ 167,410 $ 866,933 $ 787,138 $ 717,152
United States                      
Disaggregation of Revenue [Line Items]                      
Revenues                 815,405 734,748 665,793
Canada                      
Disaggregation of Revenue [Line Items]                      
Revenues                 35,031 36,728 $ 42,186
Other                      
Disaggregation of Revenue [Line Items]                      
Revenues                 16,497 15,662  
Project revenue                      
Disaggregation of Revenue [Line Items]                      
Revenues                 611,064 545,053  
O&M revenue                      
Disaggregation of Revenue [Line Items]                      
Revenues                 66,709 65,236  
Energy assets                      
Disaggregation of Revenue [Line Items]                      
Revenues                 98,042 95,776  
Other                      
Disaggregation of Revenue [Line Items]                      
Revenues                 91,118 81,073  
U.S. Regions                      
Disaggregation of Revenue [Line Items]                      
Revenues                 365,060 334,344  
U.S. Regions | United States                      
Disaggregation of Revenue [Line Items]                      
Revenues                 364,987 334,344  
U.S. Regions | Canada                      
Disaggregation of Revenue [Line Items]                      
Revenues                 0 0  
U.S. Regions | Other                      
Disaggregation of Revenue [Line Items]                      
Revenues                 73 0  
U.S. Regions | Project revenue                      
Disaggregation of Revenue [Line Items]                      
Revenues                 321,973 296,226  
U.S. Regions | O&M revenue                      
Disaggregation of Revenue [Line Items]                      
Revenues                 15,753 17,814  
U.S. Regions | Energy assets                      
Disaggregation of Revenue [Line Items]                      
Revenues                 24,897 18,442  
U.S. Regions | Other                      
Disaggregation of Revenue [Line Items]                      
Revenues                 2,437 1,862  
U.S. Federal                      
Disaggregation of Revenue [Line Items]                      
Revenues                 287,426 246,309  
U.S. Federal | United States                      
Disaggregation of Revenue [Line Items]                      
Revenues                 287,426 246,309  
U.S. Federal | Canada                      
Disaggregation of Revenue [Line Items]                      
Revenues                 0 0  
U.S. Federal | Other                      
Disaggregation of Revenue [Line Items]                      
Revenues                 0 0  
U.S. Federal | Project revenue                      
Disaggregation of Revenue [Line Items]                      
Revenues                 240,656 202,286  
U.S. Federal | O&M revenue                      
Disaggregation of Revenue [Line Items]                      
Revenues                 41,599 39,250  
U.S. Federal | Energy assets                      
Disaggregation of Revenue [Line Items]                      
Revenues                 3,652 4,062  
U.S. Federal | Other                      
Disaggregation of Revenue [Line Items]                      
Revenues                 1,519 711  
Canada                      
Disaggregation of Revenue [Line Items]                      
Revenues                 37,910 38,982  
Canada | United States                      
Disaggregation of Revenue [Line Items]                      
Revenues                 3,007 2,557  
Canada | Canada                      
Disaggregation of Revenue [Line Items]                      
Revenues                 34,830 36,425  
Canada | Other                      
Disaggregation of Revenue [Line Items]                      
Revenues                 73 0  
Canada | Project revenue                      
Disaggregation of Revenue [Line Items]                      
Revenues                 27,995 29,571  
Canada | O&M revenue                      
Disaggregation of Revenue [Line Items]                      
Revenues                 5 37  
Canada | Energy assets                      
Disaggregation of Revenue [Line Items]                      
Revenues                 3,306 2,604  
Canada | Other                      
Disaggregation of Revenue [Line Items]                      
Revenues                 6,604 6,770  
Non-Solar DG                      
Disaggregation of Revenue [Line Items]                      
Revenues                 84,683 82,655  
Non-Solar DG | United States                      
Disaggregation of Revenue [Line Items]                      
Revenues                 84,683 82,655  
Non-Solar DG | Canada                      
Disaggregation of Revenue [Line Items]                      
Revenues                 0 0  
Non-Solar DG | Other                      
Disaggregation of Revenue [Line Items]                      
Revenues                 0 0  
Non-Solar DG | Project revenue                      
Disaggregation of Revenue [Line Items]                      
Revenues                 9,221 4,550  
Non-Solar DG | O&M revenue                      
Disaggregation of Revenue [Line Items]                      
Revenues                 9,183 8,135  
Non-Solar DG | Energy assets                      
Disaggregation of Revenue [Line Items]                      
Revenues                 65,365 69,599  
Non-Solar DG | Other                      
Disaggregation of Revenue [Line Items]                      
Revenues                 914 371  
All Other                      
Disaggregation of Revenue [Line Items]                      
Revenues                 91,854 84,848  
All Other | United States                      
Disaggregation of Revenue [Line Items]                      
Revenues                 75,302 68,883  
All Other | Canada                      
Disaggregation of Revenue [Line Items]                      
Revenues                 201 303  
All Other | Other                      
Disaggregation of Revenue [Line Items]                      
Revenues                 16,351 15,662  
All Other | Project revenue                      
Disaggregation of Revenue [Line Items]                      
Revenues                 11,219 12,420  
All Other | O&M revenue                      
Disaggregation of Revenue [Line Items]                      
Revenues                 169 0  
All Other | Energy assets                      
Disaggregation of Revenue [Line Items]                      
Revenues                 822 1,069  
All Other | Other                      
Disaggregation of Revenue [Line Items]                      
Revenues                 $ 79,644 $ 71,359