Secured Financing Agreements - Outstanding, Weighted Average Borrowing Rates, Weighted Average Remaining Maturities, Average Daily Balances and the Fair Value of Collateral Pledged (Detail) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Assets Sold under Agreements to Repurchase [Line Items]
Secured financing agreements outstanding secured by:	$ 5,944,201	$ 13,427,545
Repurchase Agreements
Assets Sold under Agreements to Repurchase [Line Items]
Secured financing agreements outstanding secured by:	5,743,417	13,427,545
MBS pledged as collateral at fair value on Repurchase agreements:	7,439,374	15,399,244
Average balance of Secured financing agreements secured by:	9,431,031	14,492,386
Repurchase Agreements | Agency MBS - Residential
Assets Sold under Agreements to Repurchase [Line Items]
Secured financing agreements outstanding secured by:	77,008	6,247,275
MBS pledged as collateral at fair value on Repurchase agreements:	99,938	6,602,039
Average balance of Secured financing agreements secured by:	$ 2,551,002	$ 8,062,881
Average borrowing rate of Secured financing agreements secured by:	1.15%	2.10%
Average remaining maturity of Secured financing agreements secured by:	6 days	14 days
Average original maturity of Secured financing agreements secured by:	12 days	38 days
Repurchase Agreements | Agency CMBS
Assets Sold under Agreements to Repurchase [Line Items]
Secured financing agreements outstanding secured by:	$ 2,046,782	$ 2,013,515
MBS pledged as collateral at fair value on Repurchase agreements:	2,158,413	2,102,520
Average balance of Secured financing agreements secured by:	$ 2,142,221	$ 1,993,372
Average borrowing rate of Secured financing agreements secured by:	0.28%	2.10%
Average remaining maturity of Secured financing agreements secured by:	15 days	13 days
Average original maturity of Secured financing agreements secured by:	30 days	34 days
Repurchase Agreements | Non-agency MBS and Loans held for investment
Assets Sold under Agreements to Repurchase [Line Items]
Secured financing agreements outstanding secured by:	$ 3,619,627	$ 5,166,755
MBS pledged as collateral at fair value on Repurchase agreements:	5,181,023	6,694,685
Average balance of Secured financing agreements secured by:	$ 4,737,808	$ 4,436,133
Average borrowing rate of Secured financing agreements secured by:	4.47%	3.19%
Average remaining maturity of Secured financing agreements secured by:	606 days	255 days
Average original maturity of Secured financing agreements secured by:	628 days	279 days
Maturity 30 to 90 Days | Repurchase Agreements
Assets Sold under Agreements to Repurchase [Line Items]
Securities Sold under Agreements to Repurchase, Fair Value of Collateral	$ 330,000