Summary of Characteristics of TBA Derivatives (Detail) (TBA Derivatives, USD $) In Thousands, unless otherwise specified	9 Months Ended	12 Months Ended
	Sep. 30, 2014	Dec. 31, 2013
Derivative Instruments:
Notional	$ (500,000)	$ (1,250,000)
Implied Cost Basis	(493,193)	(1,189,531)
Implied Market Value	(492,734)	(1,182,617)
Carrying Value	459	6,914
Purchase Contracts
Derivative Instruments:
Notional		2,625,000
Implied Cost Basis		2,733,682
Implied Market Value		2,722,324
Carrying Value		(11,358)
Sales Contracts
Derivative Instruments:
Notional	(500,000)	(3,875,000)
Implied Cost Basis	(493,193)	(3,923,213)
Implied Market Value	(492,734)	(3,904,941)
Carrying Value	$ 459	$ 18,272