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	Dec. 31, 2019	Dec. 31, 2018
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase agreements outstanding secured by:	$ 13,427,545	$ 14,030,465
Repurchase Agreements
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase agreements outstanding secured by:	13,427,545	14,030,465
Average balance of Repurchase agreements secured by:	14,492,386	9,816,170
MBS pledged as collateral at fair value on Repurchase agreements:	15,399,244	15,831,007
Repurchase Agreements | Agency MBS - Residential
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase agreements outstanding secured by:	6,247,275	8,588,446
Average balance of Repurchase agreements secured by:	$ 8,062,881	$ 4,651,613
Average borrowing rate of Repurchase agreements secured by:	2.10%	2.55%
Average remaining maturity of Repurchase agreements secured by:	14 days	29 days
Average original maturity of Repurchase agreements secured by:	38 days	89 days
MBS pledged as collateral at fair value on Repurchase agreements:	$ 6,602,039	$ 9,123,369
Repurchase Agreements | Agency CMBS
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase agreements outstanding secured by:	2,013,515	1,604,826
Average balance of Repurchase agreements secured by:	$ 1,993,372	$ 1,434,030
Average borrowing rate of Repurchase agreements secured by:	2.10%	2.58%
Average remaining maturity of Repurchase agreements secured by:	13 days	33 days
Average original maturity of Repurchase agreements secured by:	34 days	84 days
MBS pledged as collateral at fair value on Repurchase agreements:	$ 2,102,520	$ 1,708,648
Repurchase Agreements | Non-agency MBS and Loans held for investment
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase agreements outstanding secured by:	5,166,755	3,837,193
Average balance of Repurchase agreements secured by:	$ 4,436,133	$ 3,730,527
Average borrowing rate of Repurchase agreements secured by:	3.19%	4.04%
Average remaining maturity of Repurchase agreements secured by:	255 days	231 days
Average original maturity of Repurchase agreements secured by:	279 days	259 days
MBS pledged as collateral at fair value on Repurchase agreements:	$ 6,694,685	$ 4,998,990
Repurchase Agreements | Residential Mortgage-Backed Securities
Assets Sold under Agreements to Repurchase [Line Items]
MBS pledged as collateral at fair value on Repurchase agreements:	$ 15,400,000	$ 15,800,000