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	Mar. 31, 2016	Dec. 31, 2015
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase agreements outstanding	$ 7,545,631	$ 7,439,339
Repurchase Agreements [Member] | Agency MBS - Residential [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase agreements outstanding	5,470,703	5,324,729
Average daily balance	$ 5,419,402	$ 5,776,980
Weighted average borrowing rate	0.76%	0.64%
Weighted average maturity	52 days	53 days
Securities pledged as collateral	$ 5,810,037	$ 5,655,410
Repurchase Agreements [Member] | Non-Agency RMBS [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase agreements outstanding	2,074,928	2,114,610
Average daily balance	$ 2,077,523	$ 1,627,909
Weighted average borrowing rate	2.69%	2.44%
Weighted average maturity	131 days	137 days
Securities pledged as collateral	$ 3,111,388	$ 3,117,875
Repurchase Agreements [Member] | Residential Mortgage-Backed Securities [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase agreements outstanding	7,545,631	7,439,339
Average daily balance	7,496,925	7,404,889
Securities pledged as collateral	$ 8,921,425	$ 8,773,285