Repurchase Agreements - Outstanding, Weighted Average Borrowing Rates, Weighted Average Remaining Maturities, Average Daily Balances and the Fair Value of Collateral Pledged (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase agreements outstanding secured by:	$ 7,146,996	$ 13,427,545
Repurchase Agreements
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase agreements outstanding secured by:	7,146,996	13,427,545
MBS pledged as collateral at fair value on Repurchase agreements:	8,341,399	15,399,244
Average balance of Repurchase agreements secured by:	11,754,793	14,492,386
Repurchase Agreements | Agency MBS - Residential
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase agreements outstanding secured by:	75,866	6,247,275
MBS pledged as collateral at fair value on Repurchase agreements:	89,561	6,602,039
Average balance of Repurchase agreements secured by:	$ 4,406,107	$ 8,062,881
Average borrowing rate of Repurchase agreements secured by:	0.94%	2.10%
Average remaining maturity of Repurchase agreements secured by:	4 days	14 days
Average original maturity of Repurchase agreements secured by:	16 days	38 days
Repurchase Agreements | Agency CMBS
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase agreements outstanding secured by:	$ 2,206,233	$ 2,013,515
MBS pledged as collateral at fair value on Repurchase agreements:	2,347,087	2,102,520
Average balance of Repurchase agreements secured by:	$ 2,112,244	$ 1,993,372
Average borrowing rate of Repurchase agreements secured by:	1.06%	2.10%
Average remaining maturity of Repurchase agreements secured by:	14 days	13 days
Average original maturity of Repurchase 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]
Repurchase agreements outstanding secured by:	$ 4,864,897	$ 5,166,755
MBS pledged as collateral at fair value on Repurchase agreements:	5,904,751	6,694,685
Average balance of Repurchase agreements secured by:	$ 5,236,442	$ 4,436,133
Average borrowing rate of Repurchase agreements secured by:	3.59%	3.19%
Average remaining maturity of Repurchase agreements secured by:	230 days	255 days
Average original maturity of Repurchase agreements secured by:	250 days	279 days