Effect of Other Derivative Contracts on the Consolidated Statements of Operations and Comprehensive Income (Loss) (Detail) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Derivative Instruments:
Unrealized Gain (Loss)		$ (822,585)	$ 98,593	$ (587,995)	$ (75,287)
Amount of Gain/(Loss) Recognized in Net Gains (Losses) on Trading Assets		108,179	4,678	(12,942)	(193,654)
Future
Derivative Instruments:
Realized Gain (Loss)				(51,205)
Unrealized Gain (Loss)				(109,974)
Amount of Gain/(Loss) Recognized in Net Gains (Losses) on Trading Assets				(161,179)
Us Treasury futures
Derivative Instruments:
Realized Gain (Loss)		(36,468)	(2,991)		(15,466)
Unrealized Gain (Loss)		(105,199)	6,455		3,631
Amount of Gain/(Loss) Recognized in Net Gains (Losses) on Trading Assets		(141,667)	3,464		(11,835)
TBA Derivatives
Derivative Instruments:
Realized Gain (Loss)	[1]	168,292	(1,864)	61,846	(46,747)
Unrealized Gain (Loss)	[1]	81,560	6,992	91,773	(8,046)
Amount of Gain/(Loss) Recognized in Net Gains (Losses) on Trading Assets	[1]	249,852	5,128	153,619	(54,793)
Interest Rate Swaption
Derivative Instruments:
Realized Gain (Loss)		(11,525)	(30,432)	(41,016)	(102,413)
Unrealized Gain (Loss)		11,519	26,518	35,634	(24,613)
Amount of Gain/(Loss) Recognized in Net Gains (Losses) on Trading Assets		$ (6)	$ (3,914)	$ (5,382)	$ (127,026)

[1] Includes options on TBA securities.