Power Purchase Agreement Programs - Schedule of VIEs (Details) $ in Thousands	3 Months Ended	9 Months Ended		12 Months Ended
	Sep. 30, 2019 MWh MW	Sep. 30, 2019 USD ($) MWh MW	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)
Variable Interest Entity [Line Items]
Distributions to Equity Investor		$ 288	$ 288
PPA II
Variable Interest Entity [Line Items]
Maximum size of installation (in megawatts) | MWh	30	30
Installed size (in megawatts) | MWh	11	11
Term of power purchase agreements (years)		21 years
Income (loss) and tax benefits allocation to Equity Investor	99.00%	99.00%
Cash allocation to Equity Investor	99.00%	99.00%
Income (loss), tax and cash allocations to Equity Investor after the flip date	5.00%	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		163,626		$ 116,942
Debt repayment—principal		$ 144,813		65,114
PPA IIIa
Variable Interest Entity [Line Items]
Maximum size of installation (in megawatts) | MWh	10	10
Installed size (in megawatts) | MWh	10	10
Term of power purchase agreements (years)		15 years
Income (loss) and tax benefits allocation to Equity Investor	99.00%	99.00%
Cash allocation to Equity Investor	99.00%	99.00%
Income (loss), tax and cash allocations to Equity Investor after the flip date	5.00%	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		4,616		4,063
Debt repayment—principal		$ 6,122		4,431
PPA IIIb
Variable Interest Entity [Line Items]
Maximum size of installation (in megawatts) | MWh	6	6
Installed size (in megawatts) | MWh	5	5
Term of power purchase agreements (years)		15 years
Income (loss) and tax benefits allocation to Equity Investor	99.00%	99.00%
Cash allocation to Equity Investor	99.00%	99.00%
Income (loss), tax and cash allocations to Equity Investor after the flip date	5.00%	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		2,109		1,807
Debt repayment—principal		$ 4,608		3,953
PPA IV
Variable Interest Entity [Line Items]
Maximum size of installation (in megawatts) | MWh	21	21
Installed size (in megawatts) | MWh	19	19
Term of power purchase agreements (years)		15 years
Income (loss) and tax benefits allocation to Equity Investor	90.00%	90.00%
Cash allocation to Equity Investor	90.00%	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		6,422		4,568
Debt repayment—principal		$ 17,314		15,543
PPA V
Variable Interest Entity [Line Items]
Maximum size of installation (in megawatts) | MWh	40	40
Installed size (in megawatts) | MWh	37	37
Term of power purchase agreements (years)		15 years
Income (loss) and tax benefits allocation to Equity Investor	99.00%	99.00%
Cash allocation to Equity Investor	90.00%	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		70,141		66,745
Debt repayment—principal		$ 8,633		$ 5,780
Write Off Of Energy Servers
Variable Interest Entity [Line Items]
Energy Server repurchased | MW	9	19