Summary of Characteristics of TBA Derivatives (Detail) (TBA Derivatives, USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2014	Dec. 31, 2013
Derivative Instruments:
Notional	$ (625,000)	$ (1,250,000)
Cost Basis	(558,442)	(1,189,531)
Market Value	(561,347)	(1,182,617)
Carrying Value	(2,905)	6,914
Purchase Contracts
Derivative Instruments:
Notional	1,500,000	2,625,000
Cost Basis	1,569,331	2,733,682
Market Value	1,568,184	2,722,324
Carrying Value	(1,147)	(11,357)
Sales Contracts
Derivative Instruments:
Notional	(2,125,000)	(3,875,000)
Cost Basis	(2,127,773)	(3,923,213)
Market Value	(2,129,531)	(3,904,941)
Carrying Value	$ (1,758)	$ 18,271