Short-Term Debt - Repurchase Agreements Balances, Weighted Average Interest Rates and Remaining Weighted Average Maturities (Detail) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2020	Dec. 31, 2019
Assets Sold Under Agreements To Repurchase [Line Items]
Repurchase agreements	$ 1,697,181	$ 3,657,873
Agency MBS
Assets Sold Under Agreements To Repurchase [Line Items]
Weighted average maturity	23 days	30 days
Repurchase agreements	$ 1,595,000
Weighted average interest rate	0.24%
Agency MBS | Collateral Pledged
Assets Sold Under Agreements To Repurchase [Line Items]
MBS pledged as collateral under the repurchase agreements and interest rate swaps	$ 1,679,841	$ 3,419,375
Agency MBS | Less than 30 days
Assets Sold Under Agreements To Repurchase [Line Items]
Repurchase agreements	$ 1,215,000	$ 1,680,000
Weighted average interest rate	0.24%	2.04%
Agency MBS | 30 days to 90 days
Assets Sold Under Agreements To Repurchase [Line Items]
Repurchase agreements	$ 380,000	$ 1,550,000
Weighted average interest rate	0.24%	1.89%
Agency MBS | Demand
Assets Sold Under Agreements To Repurchase [Line Items]
Repurchase agreements		$ 3,230,000
Weighted average interest rate		1.97%
Non-Agency MBS
Assets Sold Under Agreements To Repurchase [Line Items]
Weighted average maturity	26 days	11 days
Repurchase agreements	$ 102,181
Weighted average interest rate	2.78%
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	$ 164,741	$ 535,315
Non-Agency MBS | Less than 30 days
Assets Sold Under Agreements To Repurchase [Line Items]
Repurchase agreements	$ 81,344	$ 427,873
Weighted average interest rate	2.66%	2.80%
Non-Agency MBS | 30 days to 90 days
Assets Sold Under Agreements To Repurchase [Line Items]
Repurchase agreements	$ 20,837
Weighted average interest rate	3.24%
Non-Agency MBS | Demand
Assets Sold Under Agreements To Repurchase [Line Items]
Repurchase agreements		$ 427,873
Weighted average interest rate		2.80%
MBS
Assets Sold Under Agreements To Repurchase [Line Items]
Weighted average maturity	23 days	28 days
Weighted average interest rate after adjusting for interest rate swaps	1.24%	2.13%
Weighted average maturity after adjusting for interest rate swaps	983 days	978 days
Repurchase agreements	$ 1,697,181
Weighted average interest rate	0.39%
MBS | Collateral Pledged
Assets Sold Under Agreements To Repurchase [Line Items]
MBS pledged as collateral under the repurchase agreements and interest rate swaps	$ 1,844,582	$ 3,954,690
MBS | Less than 30 days
Assets Sold Under Agreements To Repurchase [Line Items]
Repurchase agreements	$ 1,296,344	$ 2,107,873
Weighted average interest rate	0.39%	2.20%
MBS | 30 days to 90 days
Assets Sold Under Agreements To Repurchase [Line Items]
Repurchase agreements	$ 400,837	$ 1,550,000
Weighted average interest rate	0.39%	1.89%
MBS | Demand
Assets Sold Under Agreements To Repurchase [Line Items]
Repurchase agreements		$ 3,657,873
Weighted average interest rate		2.07%