Fair Value - Estimated Fair Values and Carrying Values of Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Assets
Loans at amortized cost	$ 0	$ 45,423
Level 1
Assets
Loans at amortized cost	0	0
Liabilities
Line of credit		0
Level 1 \| Convertible senior notes
Liabilities
Secured and unsecured debt	547,300	608,491
Level 1 \| Secured debt
Liabilities
Secured and unsecured debt	0
Level 1 \| Secured and Unsecured Debt
Liabilities
Secured and unsecured debt		0
Level 1 \| Securitization bonds payable
Liabilities
Secured and unsecured debt		0
Level 1 \| Junior Subordinated Notes
Liabilities
Secured and unsecured debt	0	0
Level 2
Assets
Loans at amortized cost	0	0
Liabilities
Line of credit		50,000
Level 2 \| Convertible senior notes
Liabilities
Secured and unsecured debt	13,095	13,979
Level 2 \| Secured debt
Liabilities
Secured and unsecured debt	0
Level 2 \| Secured and Unsecured Debt
Liabilities
Secured and unsecured debt		0
Level 2 \| Securitization bonds payable
Liabilities
Secured and unsecured debt		132,815
Level 2 \| Junior Subordinated Notes
Liabilities
Secured and unsecured debt	0	0
Level 3
Assets
Loans at amortized cost	1,667,892	3,232,301
Liabilities
Line of credit		0
Level 3 \| Convertible senior notes
Liabilities
Secured and unsecured debt	0	0
Level 3 \| Secured debt
Liabilities
Secured and unsecured debt	9,218,692
Level 3 \| Secured and Unsecured Debt
Liabilities
Secured and unsecured debt		9,703,680
Level 3 \| Securitization bonds payable
Liabilities
Secured and unsecured debt		169,908
Level 3 \| Junior Subordinated Notes
Liabilities
Secured and unsecured debt	169,619	216,316
Fair Value
Assets
Loans at amortized cost	1,667,892	3,232,301
Liabilities
Line of credit		50,000
Fair Value \| Convertible senior notes
Liabilities
Secured and unsecured debt	560,395	622,470
Fair Value \| Secured debt
Liabilities
Secured and unsecured debt	9,218,692
Fair Value \| Secured and Unsecured Debt
Liabilities
Secured and unsecured debt		9,703,680
Fair Value \| Securitization bonds payable
Liabilities
Secured and unsecured debt		302,723
Fair Value \| Junior Subordinated Notes
Liabilities
Secured and unsecured debt	169,619	216,316
Carrying Value
Assets
Loans at amortized cost	1,659,217	3,178,339
Liabilities
Line of credit		50,000
Carrying Value \| Convertible senior notes
Liabilities
Secured and unsecured debt	612,150	610,331
Carrying Value \| Secured debt
Liabilities
Secured and unsecured debt	9,228,721
Carrying Value \| Secured and Unsecured Debt
Liabilities
Secured and unsecured debt		9,622,175
Carrying Value \| Securitization bonds payable
Liabilities
Secured and unsecured debt		303,709
Carrying Value \| Junior Subordinated Notes
Liabilities
Secured and unsecured debt	$ 199,086	$ 197,053

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- Definition

Fair value portion of debt instrument payable, including, but not limited to, notes payable and loans payable.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=109250915&loc=d3e13433-108611

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- Definition

Fair value portion of contractual arrangement with a lender under which borrowings can be made up to a specific amount at any point in time.

+ References

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- Definition

Fair value portion of loan receivable, including, but not limited to, mortgage loans held for investment, finance receivables held for investment, policy loans on insurance contracts.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-URI http://asc.fasb.org/subtopic&trid=2196772

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