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		6 Months Ended	12 Months Ended
	Jun. 30, 2016	Jun. 30, 2016	Dec. 31, 2015
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase agreements outstanding	$ 5,856,263	$ 5,856,263	$ 7,439,339
Repurchase Agreements | Agency MBS - Residential
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase agreements outstanding	3,393,252	$ 3,393,252	5,324,729
Average daily balance	$ 5,015,804		$ 5,776,980
Weighted average borrowing rate	0.80%	0.80%	0.64%
Weighted average maturity		41 days	53 days
Securities pledged as collateral	$ 3,637,847	$ 3,637,847	$ 5,655,410
Repurchase Agreements | Non-agency MBS
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase agreements outstanding	2,463,011	$ 2,463,011	2,114,610
Average daily balance	$ 2,164,639		$ 1,627,909
Weighted average borrowing rate	2.70%	2.70%	2.44%
Weighted average maturity		117 days	137 days
Securities pledged as collateral	$ 3,677,909	$ 3,677,909	$ 3,117,875
Repurchase Agreements | Residential Mortgage-Backed Securities
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase agreements outstanding	5,856,263	5,856,263	7,439,339
Average daily balance	7,180,443		7,404,889
Securities pledged as collateral	$ 7,315,756	$ 7,315,756	$ 8,773,285