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, 2018
Sep. 30, 2018
Jun. 30, 2018
Mar. 31, 2018
Dec. 31, 2017
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Disaggregation of Revenue [Line Items]                      
Revenues $ 217,371 $ 205,375 $ 196,982 $ 167,410 $ 211,133 $ 204,744 $ 166,665 $ 134,610 $ 787,138 $ 717,152 $ 651,227
United States                      
Disaggregation of Revenue [Line Items]                      
Revenues                 734,748 665,793 588,791
Canada                      
Disaggregation of Revenue [Line Items]                      
Revenues                 36,728 $ 42,186 $ 49,706
Other                      
Disaggregation of Revenue [Line Items]                      
Revenues                 15,662    
Project revenue                      
Disaggregation of Revenue [Line Items]                      
Revenues                 545,053    
O&M revenue                      
Disaggregation of Revenue [Line Items]                      
Revenues                 65,236    
Energy assets                      
Disaggregation of Revenue [Line Items]                      
Revenues                 95,776    
Other                      
Disaggregation of Revenue [Line Items]                      
Revenues                 81,073    
U.S. Regions                      
Disaggregation of Revenue [Line Items]                      
Revenues                 334,344    
U.S. Regions | United States                      
Disaggregation of Revenue [Line Items]                      
Revenues                 334,344    
U.S. Regions | Canada                      
Disaggregation of Revenue [Line Items]                      
Revenues                 0    
U.S. Regions | Other                      
Disaggregation of Revenue [Line Items]                      
Revenues                 0    
U.S. Regions | Project revenue                      
Disaggregation of Revenue [Line Items]                      
Revenues                 296,226    
U.S. Regions | O&M revenue                      
Disaggregation of Revenue [Line Items]                      
Revenues                 17,814    
U.S. Regions | Energy assets                      
Disaggregation of Revenue [Line Items]                      
Revenues                 18,442    
U.S. Regions | Other                      
Disaggregation of Revenue [Line Items]                      
Revenues                 1,862    
U.S. Federal                      
Disaggregation of Revenue [Line Items]                      
Revenues                 246,309    
U.S. Federal | United States                      
Disaggregation of Revenue [Line Items]                      
Revenues                 246,309    
U.S. Federal | Canada                      
Disaggregation of Revenue [Line Items]                      
Revenues                 0    
U.S. Federal | Other                      
Disaggregation of Revenue [Line Items]                      
Revenues                 0    
U.S. Federal | Project revenue                      
Disaggregation of Revenue [Line Items]                      
Revenues                 202,286    
U.S. Federal | O&M revenue                      
Disaggregation of Revenue [Line Items]                      
Revenues                 39,250    
U.S. Federal | Energy assets                      
Disaggregation of Revenue [Line Items]                      
Revenues                 4,062    
U.S. Federal | Other                      
Disaggregation of Revenue [Line Items]                      
Revenues                 711    
Canada                      
Disaggregation of Revenue [Line Items]                      
Revenues                 38,982    
Canada | United States                      
Disaggregation of Revenue [Line Items]                      
Revenues                 2,557    
Canada | Canada                      
Disaggregation of Revenue [Line Items]                      
Revenues                 36,425    
Canada | Other                      
Disaggregation of Revenue [Line Items]                      
Revenues                 0    
Canada | Project revenue                      
Disaggregation of Revenue [Line Items]                      
Revenues                 29,571    
Canada | O&M revenue                      
Disaggregation of Revenue [Line Items]                      
Revenues                 37    
Canada | Energy assets                      
Disaggregation of Revenue [Line Items]                      
Revenues                 2,604    
Canada | Other                      
Disaggregation of Revenue [Line Items]                      
Revenues                 6,770    
Non-Solar DG                      
Disaggregation of Revenue [Line Items]                      
Revenues                 82,655    
Non-Solar DG | United States                      
Disaggregation of Revenue [Line Items]                      
Revenues                 82,655    
Non-Solar DG | Canada                      
Disaggregation of Revenue [Line Items]                      
Revenues                 0    
Non-Solar DG | Other                      
Disaggregation of Revenue [Line Items]                      
Revenues                 0    
Non-Solar DG | Project revenue                      
Disaggregation of Revenue [Line Items]                      
Revenues                 4,550    
Non-Solar DG | O&M revenue                      
Disaggregation of Revenue [Line Items]                      
Revenues                 8,135    
Non-Solar DG | Energy assets                      
Disaggregation of Revenue [Line Items]                      
Revenues                 69,599    
Non-Solar DG | Other                      
Disaggregation of Revenue [Line Items]                      
Revenues                 371    
Other                      
Disaggregation of Revenue [Line Items]                      
Revenues                 84,848    
Other | United States                      
Disaggregation of Revenue [Line Items]                      
Revenues                 68,883    
Other | Canada                      
Disaggregation of Revenue [Line Items]                      
Revenues                 303    
Other | Other                      
Disaggregation of Revenue [Line Items]                      
Revenues                 15,662    
Other | Project revenue                      
Disaggregation of Revenue [Line Items]                      
Revenues                 12,420    
Other | O&M revenue                      
Disaggregation of Revenue [Line Items]                      
Revenues                 0    
Other | Energy assets                      
Disaggregation of Revenue [Line Items]                      
Revenues                 1,069    
Other | Other                      
Disaggregation of Revenue [Line Items]                      
Revenues                 $ 71,359