Indebtedness - Schedule of Debt (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Long term debt, current	$ 24,571	$ 21,529
Long term debt, noncurrent	1,473,038	1,273,416
Long term debt	1,497,609	1,294,945
Unused borrowing capacity	$ 5,406	$ 13,191
Bank line of credit \| Minimum
Debt Instrument [Line Items]
Interest rate	5.31%	4.58%
Bank line of credit \| Maximum
Debt Instrument [Line Items]
Interest rate	5.42%	4.87%
Senior \| Minimum
Debt Instrument [Line Items]
Interest rate	4.23%	3.63%
Senior \| Maximum
Debt Instrument [Line Items]
Interest rate	5.34%	4.69%
Subordinated
Debt Instrument [Line Items]
Unused borrowing capacity	$ 0	$ 27
Subordinated \| Minimum
Debt Instrument [Line Items]
Interest rate	6.71%	6.36%
Subordinated \| Maximum
Debt Instrument [Line Items]
Interest rate	7.82%	7.13%
Recourse debt
Debt Instrument [Line Items]
Long term debt, current	$ 0	$ 0
Long term debt, noncurrent	247,000	247,000
Long term debt	247,000	247,000
Unused borrowing capacity	406	406
Recourse debt \| Bank line of credit
Debt Instrument [Line Items]
Long term debt, current	0	0
Long term debt, noncurrent	247,000	247,000
Long term debt	247,000	247,000
Unused borrowing capacity	406	406
Non-recourse debt
Debt Instrument [Line Items]
Long term debt, current	24,571	21,529
Long term debt, noncurrent	1,226,038	1,026,416
Long term debt	1,250,609	1,047,945
Unused borrowing capacity	5,000	12,785
Non-recourse debt \| Senior
Debt Instrument [Line Items]
Long term debt, current	15,701	3,561
Long term debt, noncurrent	987,987	804,894
Long term debt	1,003,688	808,455
Unused borrowing capacity	5,000	12,758
Non-recourse debt \| Subordinated
Debt Instrument [Line Items]
Long term debt, current	4,774	4,301
Long term debt, noncurrent	148,900	107,187
Long term debt	153,674	111,488
Non-recourse debt \| Securitization Class A
Debt Instrument [Line Items]
Long term debt, current	3,644	3,534
Long term debt, noncurrent	79,716	82,203
Long term debt	83,360	85,737
Unused borrowing capacity	$ 0	$ 0
Interest rate	4.40%	4.40%
Non-recourse debt \| Securitization Class B
Debt Instrument [Line Items]
Long term debt, current	$ 452	$ 440
Long term debt, noncurrent	9,435	9,644
Long term debt	9,887	10,084
Unused borrowing capacity	$ 0	$ 0
Interest rate	5.38%	5.38%
Non-recourse debt \| SREC Loans
Debt Instrument [Line Items]
Long term debt, current		$ 9,693
Long term debt, noncurrent		22,488
Long term debt		32,181
Unused borrowing capacity		$ 0
Interest rate		7.28%

Contractual interest rate for funds borrowed, under the debt agreement.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(1))
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Amount of unused borrowing capacity under the long-term financing arrangement that is available to the entity as of the balance sheet date.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(b))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(16))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
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-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
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-Section S99
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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt, classified as current. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
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Amount after unamortized (discount) premium and debt issuance costs of long-term debt classified as noncurrent and excluding amounts to be repaid within one year or the normal operating cycle, if longer. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
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-Section S99
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