Quarterly report pursuant to Section 13 or 15(d)

Summary of Significant Accounting Policies - Energy Assets (Details)

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Summary of Significant Accounting Policies - Energy Assets (Details) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Dec. 31, 2017
Accounting Policies [Abstract]          
Interest costs capitalized $ 638 $ 925 $ 2,376 $ 3,452  
Minimum qualifying operating period for prorated repayments of grants     5 years    
Deferred grant income $ 6,774   $ 6,774   $ 7,188