Securitized Debt - Narrative (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Debt Instrument [Line Items]
Repurchase of securitized debt borrowings, collateralized by Non-Agency RMBS									$ 0	$ 0	$ 56,072
Loss on extinguishment of debt	$ (1,334)	$ 45	$ 0	$ 1,766	$ (8,906)	$ 19,915	$ (5,079)	$ 0	477	5,930	2,184
Net unrealized losses on financial instruments at fair value	20,664	$ (32,999)	$ (30,347)	$ (16,871)	69,793	$ 40,955	$ 37,260	$ 10,425	(59,552)	158,433	$ (193,534)
Securitized Loans | Non-agency MBS
Debt Instrument [Line Items]
Acquired securitized debt collateral outstanding principal balance									10,000
Repurchase of securitized debt borrowings, collateralized by Non-Agency RMBS									11,000
Loss on extinguishment of debt									355
Securitized Loans | Variable Interest Entities, Primary Beneficiary, Aggregated Disclosure | Non-agency MBS
Debt Instrument [Line Items]
Principal balance of debt	$ 350,000				$ 545,000				$ 350,000	$ 545,000
Weighted average cost of financing	5.21%				4.49%				5.21%	4.49%
Securitized Loans | Variable Interest Entities, Primary Beneficiary, Aggregated Disclosure | Loans Held for Investment
Debt Instrument [Line Items]
Principal balance of debt	$ 7,100,000				$ 3,800,000				$ 7,100,000	$ 3,800,000
Weighted average cost of financing	3.99%				3.24%				3.99%	3.24%
Net unrealized losses on financial instruments at fair value									$ 90,000	$ 39,000
Securitized Loans | Variable Interest Entity, Primary Beneficiary | Loans Held for Investment
Debt Instrument [Line Items]
Acquired securitized debt collateral outstanding principal balance									609,000	1,400,000
Repurchase of securitized debt borrowings, collateralized by Non-Agency RMBS									609,000	1,400,000
Loss on extinguishment of debt									$ 122	$ 6,000