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, 2016	Dec. 31, 2015
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase agreements outstanding	$ 5,600,903	$ 7,439,339
Repurchase Agreements | Agency MBS - Residential
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase agreements outstanding	3,087,734	5,324,729
Average daily balance	$ 4,159,651	$ 5,776,980
Weighted average borrowing rate	0.90%	0.64%
Weighted average maturity	32 days	53 days
Securities pledged as collateral	$ 3,334,245	$ 5,655,410
Repurchase Agreements | Non-agency MBS
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase agreements outstanding	2,513,169	2,114,610
Average daily balance	$ 2,322,683	$ 1,627,909
Weighted average borrowing rate	3.05%	2.44%
Weighted average maturity	98 days	137 days
Securities pledged as collateral	$ 3,699,621	$ 3,117,875
Repurchase Agreements | Residential Mortgage-Backed Securities
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase agreements outstanding	5,600,903	7,439,339
Average daily balance	6,482,334	7,404,889
Securities pledged as collateral	$ 7,033,866	$ 8,773,285