Outstanding Loans and Security Agreements - Schedule of Debt (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Jul. 27, 2018	Jun. 30, 2018	Jan. 18, 2018	Dec. 31, 2017	Sep. 30, 2017	Jul. 01, 2017	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2014	Jul. 31, 2014	Mar. 31, 2013	Dec. 31, 2012
Debt Instrument [Line Items]
Unpaid principal balance	$ 792,713				$ 1,008,104
Current portion of debt	21,922				20,136
Long-term portion of debt	718,889				921,205
Total	740,811				941,341
Unused Borrowing Capacity	$ 1,504				$ 1,784
Notes due July 2024, Recourse
Debt Instrument [Line Items]
Interest rate percentage	10.00%				10.00%
Term loan | Term Loan due November 2020, Recourse
Debt Instrument [Line Items]
Unpaid principal balance	$ 3,631				$ 5,000
Current portion of debt	1,686				1,690
Long-term portion of debt	1,945				3,197
Total	3,631				4,887
Non-recourse debt
Debt Instrument [Line Items]
Unpaid principal balance	358,576				372,318
Current portion of debt	20,236				18,446
Long-term portion of debt	328,358				342,050
Total	348,594				$ 360,496
Convertible promissory notes | Convertible Promissory Notes Interest Rate 5% Due December 2019 And 2020, Recourse
Debt Instrument [Line Items]
Interest rate percentage					5.00%
Convertible promissory notes | Convertible Promissory Notes due December 2019 and 2020, Recourse
Debt Instrument [Line Items]
Unpaid principal balance	34,273	$ 221,600
Current portion of debt	0
Long-term portion of debt	34,273				$ 244,700
Total	$ 34,273
Interest rate percentage	5.00%			5.00%
Convertible promissory notes | 6% Convertible Promissory Notes
Debt Instrument [Line Items]
Unpaid principal balance	$ 296,233				286,069
Current portion of debt	0				0
Long-term portion of debt	258,862				236,724
Total	$ 258,862				$ 236,724
Interest rate percentage	6.00%				6.00%		6.00%
Notes | Notes due July 2024, Recourse
Debt Instrument [Line Items]
Unpaid principal balance	$ 100,000				$ 100,000
Current portion of debt	0				0
Long-term portion of debt	95,451				94,517
Total	95,451				94,517
Interest rate percentage						10.00%
Recourse debt
Debt Instrument [Line Items]
Unpaid principal balance	434,137				635,786
Current portion of debt	1,686				1,690
Long-term portion of debt	390,531				579,155
Total	392,217				580,845
PPA Company II | Senior secured notes | Senior Secured Notes due March 2025, Non-Recourse
Debt Instrument [Line Items]
Unpaid principal balance	82,624				91,086
Current portion of debt	11,840				11,389
Long-term portion of debt	69,560				78,175
Total	$ 81,400				$ 89,564
Interest rate percentage	5.22%				5.20%							5.22%
PPA Company IIIa | Term loan | Term Loan due September 2028, Non-Recourse
Debt Instrument [Line Items]
Unpaid principal balance	$ 40,937				$ 41,927
Current portion of debt	1,737				1,389
Long-term portion of debt	34,765				35,551
Total	$ 36,502				$ 36,940
Interest rate percentage	7.50%				7.50%								7.50%
PPA Company IIIb | Term loan | Senior Secured Notes due October 2020, Non-Recourse
Debt Instrument [Line Items]
Unpaid principal balance	$ 24,934				$ 25,599
Current portion of debt	866				876
Long-term portion of debt	23,155				23,488
Total	24,021				24,364
PPA Company IV | Senior secured notes | Senior Secured Notes due March 2030, Non-Recourse
Debt Instrument [Line Items]
Unpaid principal balance	83,993				85,303
Current portion of debt	2,308				1,846
Long-term portion of debt	80,529				82,186
Total	$ 82,837				$ 84,032
Interest rate percentage	6.07%				6.10%						6.07%
PPA Company V | Term loan | Term Loan due December 2021, Non-Recourse
Debt Instrument [Line Items]
Unpaid principal balance	$ 126,088				$ 128,403
Current portion of debt	3,485				2,946
Long-term portion of debt	120,349				122,650
Total	$ 123,834				$ 125,596
PPA Company V | Letters of Credit
Debt Instrument [Line Items]
Interest rate percentage	2.25%				2.25%
Letters of Credit
Debt Instrument [Line Items]
Unused Borrowing Capacity	$ 1,504				$ 1,784
Letters of Credit | PPA Company V
Debt Instrument [Line Items]
Unused Borrowing Capacity	1,504				1,784
Affiliated entity
Debt Instrument [Line Items]
Total	68,700				$ 107,000
Affiliated entity | Convertible Promissory Notes Interest Rate 8% Due December 2019 And 2020, Recourse
Debt Instrument [Line Items]
Interest rate percentage					8.00%
Affiliated entity | Convertible promissory notes | Convertible Promissory Notes Interest Rate 8% Due December 2019 And 2020, Recourse
Debt Instrument [Line Items]
Unpaid principal balance					$ 244,717
Current portion of debt					0
Long-term portion of debt					244,717
Total					244,717
Affiliated entity | Convertible promissory notes | Convertible Promissory Notes due December 2019 and 2020, Recourse
Debt Instrument [Line Items]
Unpaid principal balance			$ 39,900		38,300
Interest rate percentage										8.00%
Affiliated entity | Convertible promissory notes | 6% Convertible Promissory Notes
Debt Instrument [Line Items]
Unpaid principal balance	27,700				26,800
Interest rate percentage							6.00%	6.00%	5.00%
Affiliated entity | PPA Company IIIa | Term loan | Term Loan due September 2028, Non-Recourse
Debt Instrument [Line Items]
Unpaid principal balance	$ 40,900				$ 41,900