Repurchase Agreements Balances, Weighted Average Interest Rates and Remaining Weighted Average Maturities (Detail) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2017	Dec. 31, 2016
Assets Sold Under Agreements To Repurchase [Line Items]
Repurchase agreements	$ 3,902,453	$ 3,911,015
Agency MBS
Assets Sold Under Agreements To Repurchase [Line Items]
Weighted average maturity	27 days	39 days
MBS pledged as collateral under the repurchase agreements and interest rate swaps	$ 3,690,344	$ 3,707,062
Repurchase agreements	$ 3,460,000	$ 3,500,000
Weighted average interest rate	1.20%	0.89%
Agency MBS | Less than 30 days
Assets Sold Under Agreements To Repurchase [Line Items]
Repurchase agreements	$ 2,265,000	$ 1,405,000
Weighted average interest rate	1.20%	0.85%
Agency MBS | 30 days to 90 days
Assets Sold Under Agreements To Repurchase [Line Items]
Repurchase agreements	$ 1,195,000	$ 2,095,000
Weighted average interest rate	1.19%	0.92%
Non-Agency MBS
Assets Sold Under Agreements To Repurchase [Line Items]
Weighted average maturity	17 days	17 days
MBS pledged as collateral under the repurchase agreements and interest rate swaps	$ 558,995	$ 525,169
Repurchase agreements	$ 442,453	$ 411,015
Weighted average interest rate	2.65%	2.27%
Non-Agency MBS | Less than 30 days
Assets Sold Under Agreements To Repurchase [Line Items]
Repurchase agreements	$ 381,779	$ 411,015
Weighted average interest rate	2.66%	2.27%
Non-Agency MBS | 30 days to 90 days
Assets Sold Under Agreements To Repurchase [Line Items]
Repurchase agreements	$ 60,674
Weighted average interest rate	2.61%
MBS
Assets Sold Under Agreements To Repurchase [Line Items]
Weighted average maturity	26 days	37 days
Weighted average interest rate after adjusting for interest rate swaps	1.57%	1.31%
Weighted average maturity after adjusting for interest rate swaps	524 days	488 days
MBS pledged as collateral under the repurchase agreements and interest rate swaps	$ 4,249,339	$ 4,232,231
Repurchase agreements	$ 3,902,453	$ 3,911,015
Weighted average interest rate	1.36%	1.04%
MBS | Less than 30 days
Assets Sold Under Agreements To Repurchase [Line Items]
Repurchase agreements	$ 2,646,779	$ 1,816,015
Weighted average interest rate	1.41%	1.17%
MBS | 30 days to 90 days
Assets Sold Under Agreements To Repurchase [Line Items]
Repurchase agreements	$ 1,255,674	$ 2,095,000
Weighted average interest rate	1.26%	0.92%