Summary of Significant Accounting Policies - Deferred Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Deferred Revenue Arrangement [Line Items]
Power purchase or lease agreement term			20 years
Deferred revenue	$ 580,042		$ 580,042		$ 564,852	$ 525,400
Deferred revenue, revenue recognized	13,000	$ 11,500	25,800	$ 23,000
Contract with customer, liability	4,400,000		$ 4,400,000
Revenue expected to recognize over next twelve months, percent			6.00%
Revenue recognized, term			20 years
Revenue recognized, term, existing deferred revenue			10 years
Under Customer Agreements:
Deferred Revenue Arrangement [Line Items]
Deferred revenue	564,261		$ 564,261		548,280
Under Customer Agreements: | Payments received
Deferred Revenue Arrangement [Line Items]
Deferred revenue	530,010		530,010		517,544
Under Customer Agreements: | Financing component balance
Deferred Revenue Arrangement [Line Items]
Deferred revenue	34,251		34,251		30,736
Under SREC contracts:
Deferred Revenue Arrangement [Line Items]
Deferred revenue	15,781		15,781		16,572
Under SREC contracts: | Payments received
Deferred Revenue Arrangement [Line Items]
Deferred revenue	13,916		13,916		14,805
Under SREC contracts: | Financing component balance
Deferred Revenue Arrangement [Line Items]
Deferred revenue	$ 1,865		$ 1,865		$ 1,767