Borrowings - Schedule of Borrowings (Detail) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Repurchase Agreements:
Amount outstanding	$ 14,088,838	$ 11,160,669
Weighted average interest rate	1.57%	1.23%
Weighted average remaining maturity	19 days	30 days
Secured Loans
Amount outstanding	$ 1,650,000	$ 1,650,000
Total Borrowings
Carrying value of exchangeable senior notes	157,380	397,041
Secured Debt, Excluding Asset-Backed Securities
Total Borrowings
Amount outstanding	$ 15,896,660	$ 13,210,669
Weighted average interest rate	1.58%	1.28%
Weighted average remaining maturity	269 days	373 days
Agency RMBS
Repurchase Agreements:
Amount outstanding	$ 11,115,979	$ 8,148,220
Weighted average interest rate	1.32%	0.93%
Weighted average remaining maturity	18 days	32 days
Non-Agency RMBS
Repurchase Agreements:
Amount outstanding	$ 1,035,759	$ 1,519,859
Weighted average interest rate	2.57%	2.06%
Weighted average remaining maturity	30 days	28 days
GSE CRT
Repurchase Agreements:
Amount outstanding	$ 653,156	$ 547,872
Weighted average interest rate	2.62%	2.25%
Weighted average remaining maturity	21 days	16 days
CMBS
Repurchase Agreements:
Amount outstanding	$ 1,283,944	$ 944,718
Weighted average interest rate	2.41%	1.86%
Weighted average remaining maturity	11 days	16 days
Secured Loans
Secured Loans
Amount outstanding	$ 1,650,000	$ 1,650,000
Weighted average interest rate	1.37%	0.74%
Weighted average remaining maturity	2409 days	2682 days
Exchangeable Senior Notes
Exchangeable Senior Notes
Amount outstanding		$ 400,000
Weighted average interest rate	5.00%	5.00%
Weighted average remaining maturity	166 days	439 days
Total Borrowings
Carrying value of exchangeable senior notes	$ 157,822
Debt issuance costs	$ 442	$ 3,000