Revenue - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	$ 139,940	$ 107,760	$ 251,599	$ 208,593
Electricity Sales Under PSA [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	17,414	16,580	38,100	35,401
Electricity Sales To Market [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	4,536	3,116	6,729	6,515
Renewable Energy Credits Sales [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	3,398	2,044	5,345	4,326
Electricity And REC Sales [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	25,348	21,740	50,174	46,242
Other Electricity Sales [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	110,603	83,996	187,924	157,928
Other Revenue, Related Party [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	$ 3,989	$ 2,024	$ 13,501	$ 4,423