Securitization Transactions Narrative (Details) - USD ($) $ in Thousands		9 Months Ended
		Sep. 30, 2018		Dec. 31, 2017
Securitization Transactions [Line Items]
Face amount of notes distributed from Risk Retention Vehicle		$ 5,600
Investments Fair Value		$ 2,670,069		$ 2,071,707
Minimum economic interest required to be retained		5.00%
Proceeds received from equity investment in risk retention vehicle (liquidation)		$ 5,700
Participation in Multi-Seller Consumer Loan Securitization [Member]
Securitization Transactions [Line Items]
Aggregate unpaid principal balance of loans sold		$ 124,000
Company's share of loans sold		51.00%
Co-participants share of loans sold		49.00%
Gain (Loss) on Securitization of Financial Assets		$ (100)
Senior notes issued in securitization transaction		87,000
Subordinated notes issued in securitization transaction		$ 18,700
Company's initial percentage ownership of jointly owned entity		51.00%
Affiliated co-participants initial percentage ownership of jointly owned entity		49.00%
Company's current percentage ownership of jointly owned entity		75.00%		75.00%
CLO I Securitization [Member]
Securitization Transactions [Line Items]
Gain (Loss) on Securitization of Financial Assets		$ 200
Percentage non-controlling ownership of related party		25.00%
The Company [Member] | CLO I Securitization [Member]
Securitization Transactions [Line Items]
Corporate loans transferred into CLO securitization		$ 62,000
Affiliated Entity [Member] | CLO I Securitization [Member]
Securitization Transactions [Line Items]
Corporate loans transferred into CLO securitization		141,700
Collateralized Loan Obligations [Member]
Securitization Transactions [Line Items]
Investments Fair Value		156,087	[1],[2]	$ 210,816	[3],[4]
Collateralized Loan Obligations [Member] | CLO I Securitization [Member]
Securitization Transactions [Line Items]
Investments Fair Value	[5],[6],[7]	0		24,299	[8]
Collateralized Loan Obligations [Member] | CLO II Securitization [Member]
Securitization Transactions [Line Items]
Investments Fair Value	[7],[8]	16,537		13,395
Collateralized Loan Obligations [Member] | Distributed from Risk Retention Vehicle [Member]
Securitization Transactions [Line Items]
Investments Fair Value		4,900
Corporate Equity Investments [Member]
Securitization Transactions [Line Items]
Investments Fair Value		39,824	[1],[2]	37,465	[3],[4]
Corporate Equity Investments [Member] | CLO I Securitization [Member]
Securitization Transactions [Line Items]
Investments Fair Value	[3],[4],[9]			$ 5,000
Non-QM loan securitization [Member]
Securitization Transactions [Line Items]
Notes issued by non-QM securitization trust		$ 141,200
Economic interest retained		5.10%
Cost of notes purchased from securitization trust		$ 700
Threshold for exercising Optional Redemption		30.00%
Percentage used to calculate servicing administrator fee		0.03%
Non-QM loan securitization [Member] | The Company [Member]
Securitization Transactions [Line Items]
Non-QM loans transferred into securitization trust		$ 141,200

[1] See Note 2 and Note 3 in Notes to Consolidated Financial Statements.

[2] The table below shows the Company's long investment ratings from Moody's, Standard and Poor's, or Fitch, as well as the Company's long investments that were unrated but guaranteed by the Federal National Mortgage Association, the Federal Home Loan Mortgage Corporation, or the Government National Mortgage Association. Ratings tend to be a lagging credit indicator; as a result, the credit quality of the Company's long investment holdings may be lower than the credit quality implied based on the ratings listed below. In situations where an investment has a split rating, the lowest provided rating is used. The ratings descriptions include ratings qualified with a "+," "-," "1," "2," or "3."Rating Description Percent of EquityUnrated but Agency-Guaranteed 202.37%Aaa/AAA/AAA 0.69%Aa/AA/AA 0.90%A/A/A 5.98%Baa/BBB/BBB 6.06%Ba/BB/BB or below 61.53%Unrated 155.45%

[3] See Note 2 and Note 3 in Notes to Consolidated Financial Statements.

[4] The table below shows the ratings on the Company's long investments from Moody's, Standard and Poor's, or Fitch, as well as the Company's long investments that were unrated but guaranteed by the Federal National Mortgage Association, the Federal Home Loan Mortgage Corporation, or the Government National Mortgage Association. Ratings tend to be a lagging credit indicator; as a result, the credit quality of the Company's long investment holdings may be lower than the credit quality implied based on the ratings listed below. In situations where an investment has a split rating, the lowest provided rating is used. The ratings descriptions include ratings qualified with a "+," "-," "1," "2," or "3."Rating Description Percent of EquityUnrated but Agency-Guaranteed 160.32%A/A/A 0.81%Baa/BBB/BBB 2.62%Ba/BB/BB or below 68.03%Unrated 101.85%

[5] Excludes the Company's equity investment in the CLO I Risk Retention Vehicle, as discussed above.

[6] In August 2018, the notes originally issued by the CLO I Issuer in 2017 were fully redeemed, and the CLO I Issuer simultaneously issued new notes in conjunction with this full redemption.

[7] Included on the Company's Consolidated Condensed Schedule of Investments in Collateralized Loan Obligations.

[8] Includes secured and unsecured subordinated notes.

[9] Represents the Company's investment in a related party. See Note 9 to the Notes to Consolidated Financial Statements.