Securitization Transactions (Schedule of CLO Securitization Transactions) (Details) - USD ($) $ in Thousands		3 Months Ended		12 Months Ended
		Mar. 31, 2019		Dec. 31, 2018
Securitization Transactions [Line Items]
TBA securities, at fair value				$ 2,939,311
CLO I Securitization [Member]
Securitization Transactions [Line Items]
CLO Pricing Date	[1],[2]	2018-08		2017-05
CLO Closing Date	[1],[2]	2018-08		2017-06
Total Face Amount of Notes Issued	[2]	$ 461,840		$ 373,550	[1]
Face Amount of Notes Initially Purchased	[2],[3]	36,579		36,606	[1]
Aggregate Purchase Price of Notes Initially Purchased	[2]	$ 25,622		$ 35,926	[1]
Reset CLO I Securitization [Member]
Securitization Transactions [Line Items]
CLO Pricing Date	[2]			2018-08
CLO Closing Date	[2]			2018-08
Total Face Amount of Notes Issued	[2]			$ 461,840
Face Amount of Notes Initially Purchased	[2],[3]			36,579
Aggregate Purchase Price of Notes Initially Purchased	[2]			$ 25,622
CLO II Securitization [Member]
Securitization Transactions [Line Items]
CLO Pricing Date		2017-12		2017-12
CLO Closing Date		2018-01		2018-01
Total Face Amount of Notes Issued		$ 452,800		$ 452,800
Face Amount of Notes Initially Purchased	[4]	18,223		18,223
Aggregate Purchase Price of Notes Initially Purchased		$ 16,621		$ 16,621
CLO III Securitization [Member]
Securitization Transactions [Line Items]
CLO Pricing Date		2018-06		2018-06
CLO Closing Date		2018-07		2018-07
Total Face Amount of Notes Issued		$ 407,100		$ 407,100
Face Amount of Notes Initially Purchased	[4]	35,480		35,480
Aggregate Purchase Price of Notes Initially Purchased		$ 32,394		32,394
CLO IV Securitization [Member]
Securitization Transactions [Line Items]
CLO Pricing Date		2019-02
CLO Closing Date		2019-03
Total Face Amount of Notes Issued		$ 478,488
Face Amount of Notes Initially Purchased	[4]	12,700
Aggregate Purchase Price of Notes Initially Purchased		10,618
CLOs
Securitization Transactions [Line Items]
TBA securities, at fair value				123,893
CLOs | CLO I Securitization [Member]
Securitization Transactions [Line Items]
TBA securities, at fair value	[2],[5]	17,742	[6]	0	[1]
CLOs | Reset CLO I Securitization [Member]
Securitization Transactions [Line Items]
TBA securities, at fair value	[2],[5],[6]			16,973
CLOs | CLO II Securitization [Member]
Securitization Transactions [Line Items]
TBA securities, at fair value	[5],[6]	14,931		14,721
CLOs | CLO III Securitization [Member]
Securitization Transactions [Line Items]
TBA securities, at fair value	[5],[7]	19,561		$ 19,071
CLOs | CLO IV Securitization [Member]
Securitization Transactions [Line Items]
TBA securities, at fair value	[5],[7]	$ 10,496

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

[2] 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.

[3] The Company purchased secured and unsecured subordinated notes.

[4] The Company purchased secured senior and secured and unsecured subordinated notes.

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

[6] Includes secured and unsecured subordinated notes.

[7] Includes secured senior and secured and unsecured subordinated notes.