Power Purchase Agreement Programs - Schedule of VIEs (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($) MWh	Dec. 31, 2017 USD ($) MWh	Dec. 31, 2016 USD ($) MWh
Variable Interest Entity [Line Items]
Distributions to Equity Investor	$ (576)	$ (576)	$ (7,287)
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	5
Company cash contributions	$ 180,699	$ 180,699	$ 180,699
Company non-cash contributions	0	0	0
Equity Investor cash contributions	100,000	100,000	100,000
Distributions to Equity Investor	(81,016)	(81,016)	(81,016)
Debt financing	0	0	0
Debt repayment—principal	$ 0	$ 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	30
Company cash contributions	$ 22,442	$ 22,442	$ 22,442
Company non-cash contributions	0	0	0
Equity Investor cash contributions	139,993	139,993	139,993
Distributions to Equity Investor	(116,942)	(111,296)	(107,336)
Debt financing	144,813	144,813	144,813
Debt repayment—principal	$ (65,114)	$ (53,726)	$ (39,759)
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	10
Company cash contributions	$ 32,223	$ 32,223	$ 32,223
Company non-cash contributions	8,655	8,655	8,655
Equity Investor cash contributions	36,967	36,967	36,967
Distributions to Equity Investor	(4,063)	(3,324)	(2,584)
Debt financing	44,968	44,968	44,968
Debt repayment—principal	$ (4,431)	$ (3,041)	$ (2,129)
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	5
Company cash contributions	$ 22,658	$ 22,658	$ 22,658
Company non-cash contributions	2,082	2,082	2,082
Equity Investor cash contributions	20,152	20,152	20,152
Distributions to Equity Investor	(1,807)	(1,404)	(1,002)
Debt financing	28,676	28,676	28,676
Debt repayment—principal	$ (3,953)	$ (3,077)	$ (2,356)
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	19
Company cash contributions	$ 11,669	$ 11,669	$ 11,669
Company non-cash contributions	0	0	0
Equity Investor cash contributions	84,782	84,782	84,782
Distributions to Equity Investor	(4,568)	(2,565)	(180)
Debt financing	99,000	99,000	99,000
Debt repayment—principal	$ (15,543)	$ (13,697)	$ (12,426)
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	37
Company cash contributions	$ 27,932	$ 27,932	$ 27,932
Company non-cash contributions	0	0	0
Equity Investor cash contributions	227,344	227,344	213,692
Distributions to Equity Investor	(66,745)	(60,286)	(50,827)
Debt financing	131,237	131,237	131,237
Debt repayment—principal	$ (5,780)	$ (2,834)	$ 0