Power Purchase Agreement Programs - Additional Information (Details) $ in Thousands	6 Months Ended
	Jun. 30, 2018 USD ($) variable_interest_entity	Dec. 31, 2017 USD ($)
Variable Interest Entity [Line Items]
Standard warranty period	1 year
Number of variable interest entities | variable_interest_entity	6
Redeemable noncontrolling interest | $	$ 54,940	$ 58,154
Minimum
Variable Interest Entity [Line Items]
Term of power purchase agreements (years)	10 years
Maximum
Variable Interest Entity [Line Items]
Term of power purchase agreements (years)	21 years