Power Purchase Agreement Programs - Schedule of VIEs (Details) $ in Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2018 USD ($) MWh	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($) MWh
Variable Interest Entity [Line Items]
Distributions to Equity Investor	$ (288)	$ (855)
PPA I
Variable Interest Entity [Line Items]
Maximum size of installation (in megawatts) | MWh	25
Term of power purchase agreements (years)	10 years
Income (loss) and tax benefits allocation to Equity Investor	99.00%
Cash allocation to Equity Investor	80.00%
Income (loss), tax and cash allocations to Equity Investor after the flip date	22.00%
Installed size (in megawatts) | MWh	5		5
Company cash contributions	$ 180,699		$ 180,699
Company non-cash contributions	0		0
Equity Investor cash contributions	100,000		100,000
Distributions to Equity Investor	(81,016)		(81,016)
Debt financing	0		0
Debt repayment—principal	$ 0		$ 0
PPA Company II
Variable Interest Entity [Line Items]
Maximum size of installation (in megawatts) | MWh	30
Term of power purchase agreements (years)	21 years
Income (loss) and tax benefits allocation to Equity Investor	99.00%
Cash allocation to Equity Investor	99.00%
Income (loss), tax and cash allocations to Equity Investor after the flip date	5.00%
Installed size (in megawatts) | MWh	30		30
Company cash contributions	$ 22,442		$ 22,442
Company non-cash contributions	0		0
Equity Investor cash contributions	139,993		139,993
Distributions to Equity Investor	(116,942)		(111,296)
Debt financing	144,813		144,813
Debt repayment—principal	$ (62,188)		$ (53,726)
PPA Company IIIa
Variable Interest Entity [Line Items]
Maximum size of installation (in megawatts) | MWh	10
Term of power purchase agreements (years)	15 years
Income (loss) and tax benefits allocation to Equity Investor	99.00%
Cash allocation to Equity Investor	99.00%
Income (loss), tax and cash allocations to Equity Investor after the flip date	5.00%
Installed size (in megawatts) | MWh	10		10
Company cash contributions	$ 32,223		$ 32,223
Company non-cash contributions	8,655		8,655
Equity Investor cash contributions	36,967		36,967
Distributions to Equity Investor	(3,877)		(3,324)
Debt financing	44,968		44,968
Debt repayment—principal	$ (4,031)		$ (3,041)
PPA Company IIIb
Variable Interest Entity [Line Items]
Maximum size of installation (in megawatts) | MWh	6
Term of power purchase agreements (years)	15 years
Income (loss) and tax benefits allocation to Equity Investor	99.00%
Cash allocation to Equity Investor	99.00%
Income (loss), tax and cash allocations to Equity Investor after the flip date	5.00%
Installed size (in megawatts) | MWh	5		5
Company cash contributions	$ 22,658		$ 22,658
Company non-cash contributions	2,082		2,082
Equity Investor cash contributions	20,152		20,152
Distributions to Equity Investor	(1,706)		(1,404)
Debt financing	28,676		28,676
Debt repayment—principal	$ (3,742)		$ (3,077)
PPA Company IV
Variable Interest Entity [Line Items]
Maximum size of installation (in megawatts) | MWh	21
Term of power purchase agreements (years)	15 years
Income (loss) and tax benefits allocation to Equity Investor	90.00%
Cash allocation to Equity Investor	90.00%
Installed size (in megawatts) | MWh	19		19
Company cash contributions	$ 11,669		$ 11,669
Company non-cash contributions	0		0
Equity Investor cash contributions	84,782		84,782
Distributions to Equity Investor	(4,568)		(2,565)
Debt financing	99,000		99,000
Debt repayment—principal	$ (15,007)		$ (13,697)
PPA Company V
Variable Interest Entity [Line Items]
Maximum size of installation (in megawatts) | MWh	40
Term of power purchase agreements (years)	15 years
Income (loss) and tax benefits allocation to Equity Investor	99.00%
Cash allocation to Equity Investor	90.00%
Installed size (in megawatts) | MWh	37		37
Company cash contributions	$ 27,932		$ 27,932
Company non-cash contributions	0		0
Equity Investor cash contributions	227,344		227,344
Distributions to Equity Investor	(65,688)		(60,286)
Debt financing	131,237		131,237
Debt repayment—principal	$ (5,148)		$ (2,834)