Effect of Other Derivative Contracts on the Consolidated Statements of Operations and Comprehensive Income (Loss) (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Derivative Instruments:
Unrealized Gain (Loss)	$ (948,755)		$ 2,002,200		$ (32,219)
Amount of Gain/(Loss) Recognized in Net Gains (Losses) on Trading Assets	(251,123)		(19,171)
TBA Derivatives
Derivative Instruments:
Realized Gain (Loss)	(60,091)	[1]	33,728	[1]
Unrealized Gain (Loss)	(12,763)	[1]	6,630	[1]
Amount of Gain/(Loss) Recognized in Net Gains (Losses) on Trading Assets	(72,854)	[1]	40,358	[1]
Interest Rate Swaption
Derivative Instruments:
Realized Gain (Loss)	(121,345)		(2,697)
Unrealized Gain (Loss)	(20,167)		(15,467)
Amount of Gain/(Loss) Recognized in Net Gains (Losses) on Trading Assets	(141,512)		(18,164)
Us Treasury futures
Derivative Instruments:
Realized Gain (Loss)	(30,056)		(38,514)
Unrealized Gain (Loss)	(6,701)		(2,851)
Amount of Gain/(Loss) Recognized in Net Gains (Losses) on Trading Assets	$ (36,757)		$ (41,365)

[1] Includes options on TBA securities.