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