Power Purchase Agreement Programs - Schedule of VIEs (Details) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2019 USD ($) MWh	Mar. 31, 2018 USD ($)	Dec. 31, 2018 USD ($)
Variable Interest Entity [Line Items]
Distributions to Equity Investor	$ 282	$ 282
PPA Company II
Variable Interest Entity [Line Items]
Maximum size of installation (in megawatts) | MWh	30
Installed size (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%
Company cash contributions	$ 22,442
Company non-cash contributions	0
Equity Investor cash contributions	139,993
Debt financing	144,813
Distributions to Equity Investor	116,942		$ 116,942
Debt repayment—principal	$ 67,985		65,114
PPA Company IIIa
Variable Interest Entity [Line Items]
Maximum size of installation (in megawatts) | MWh	10
Installed size (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%
Company cash contributions	$ 32,223
Company non-cash contributions	8,655
Equity Investor cash contributions	36,967
Debt financing	44,968
Distributions to Equity Investor	4,246		4,063
Debt repayment—principal	$ 5,209		4,431
PPA Company IIIb
Variable Interest Entity [Line Items]
Maximum size of installation (in megawatts) | MWh	6
Installed size (in megawatts) | MWh	5
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%
Company cash contributions	$ 22,658
Company non-cash contributions	2,082
Equity Investor cash contributions	20,152
Debt financing	28,676
Distributions to Equity Investor	1,907		1,807
Debt repayment—principal	$ 4,238		3,953
PPA Company IV
Variable Interest Entity [Line Items]
Maximum size of installation (in megawatts) | MWh	21
Installed size (in megawatts) | MWh	19
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%
Company cash contributions	$ 11,669
Company non-cash contributions	0
Equity Investor cash contributions	84,782
Debt financing	99,000
Distributions to Equity Investor	4,982		4,568
Debt repayment—principal	$ 16,111		15,543
PPA Company V
Variable Interest Entity [Line Items]
Maximum size of installation (in megawatts) | MWh	40
Installed size (in megawatts) | MWh	37
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%
Company cash contributions	$ 27,932
Company non-cash contributions	0
Equity Investor cash contributions	227,344
Debt financing	131,237
Distributions to Equity Investor	68,944		66,745
Debt repayment—principal	$ 6,644		$ 5,780