Summary of Characteristics of TBA Derivatives (Detail) (TBA Derivatives, USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Derivative Instruments:
Notional	$ (375,000)	$ (1,250,000)
Implied Cost Basis	(375,430)	(1,189,531)
Implied Market Value	(379,688)	(1,182,617)
Carrying Value	(4,258)	6,914
Purchase Contracts
Derivative Instruments:
Notional		2,625,000
Implied Cost Basis		2,733,682
Implied Market Value		2,722,324
Carrying Value		(11,357)
Sales Contracts
Derivative Instruments:
Notional	(375,000)	(3,875,000)
Implied Cost Basis	(375,430)	(3,923,213)
Implied Market Value	(379,688)	(3,904,941)
Carrying Value	$ (4,258)	$ 18,271