Repurchase Agreements Balances, Weighted Average Interest Rates and Remaining Weighted Average Maturities (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Assets Sold Under Agreements To Repurchase [Line Items]
Repurchase agreements	$ 3,657,873	$ 3,811,627
Agency MBS
Assets Sold Under Agreements To Repurchase [Line Items]
Weighted average maturity	30 days	35 days
Repurchase agreements	$ 3,230,000	$ 3,235,000
Weighted average interest rate	1.97%	2.52%
Agency MBS | Collateral Pledged
Assets Sold Under Agreements To Repurchase [Line Items]
MBS pledged as collateral under the repurchase agreements and interest rate swaps	$ 3,419,375	$ 3,433,252
Agency MBS | Less than 30 days
Assets Sold Under Agreements To Repurchase [Line Items]
Repurchase agreements	$ 1,680,000	$ 1,510,000
Weighted average interest rate	2.04%	2.46%
Agency MBS | 30 days to 90 days
Assets Sold Under Agreements To Repurchase [Line Items]
Repurchase agreements	$ 1,550,000	$ 1,725,000
Weighted average interest rate	1.89%	2.57%
Non-Agency MBS
Assets Sold Under Agreements To Repurchase [Line Items]
Weighted average maturity	11 days	13 days
Repurchase agreements	$ 427,873	$ 576,627
Weighted average interest rate	2.80%	3.55%
Non-Agency MBS | Collateral Pledged
Assets Sold Under Agreements To Repurchase [Line Items]
MBS pledged as collateral under the repurchase agreements and interest rate swaps	$ 535,315	$ 726,428
Non-Agency MBS | Less than 30 days
Assets Sold Under Agreements To Repurchase [Line Items]
Repurchase agreements	$ 427,873	$ 576,627
Weighted average interest rate	2.80%	3.55%
MBS
Assets Sold Under Agreements To Repurchase [Line Items]
Weighted average maturity	28 days	32 days
Weighted average interest rate after adjusting for interest rate swaps	2.13%	2.23%
Weighted average maturity after adjusting for interest rate swaps	978 days	1217 days
Repurchase agreements	$ 3,657,873	$ 3,811,627
Weighted average interest rate	2.07%	2.67%
MBS | Collateral Pledged
Assets Sold Under Agreements To Repurchase [Line Items]
MBS pledged as collateral under the repurchase agreements and interest rate swaps	$ 3,954,690	$ 4,159,680
MBS | Less than 30 days
Assets Sold Under Agreements To Repurchase [Line Items]
Repurchase agreements	$ 2,107,873	$ 2,086,627
Weighted average interest rate	2.20%	2.76%
MBS | 30 days to 90 days
Assets Sold Under Agreements To Repurchase [Line Items]
Repurchase agreements	$ 1,550,000	$ 1,725,000
Weighted average interest rate	1.89%	2.57%