Effect of Other Derivative Contracts on the Consolidated Statements of Operations and Comprehensive Income (Loss) (Detail) - USD ($) $ in Thousands		3 Months Ended		12 Months Ended
		Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015
Derivative Instruments:
Unrealized Gain (Loss)		$ (1,031,720)	$ (466,202)
Amount of Gain/(Loss) Recognized in Net Gains (Losses) on Trading Assets		125,174		$ (6,896)
Us Treasury futures
Derivative Instruments:
Realized Gain (Loss)		(130,994)		(5,506)
Unrealized Gain (Loss)		(48,120)		(58,126)
Amount of Gain/(Loss) Recognized in Net Gains (Losses) on Trading Assets		(179,114)		(63,632)
TBA Derivatives
Derivative Instruments:
Realized Gain (Loss)	[1]	219,993		(55,644)
Unrealized Gain (Loss)	[1]	84,295		117,188
Amount of Gain/(Loss) Recognized in Net Gains (Losses) on Trading Assets	[1]	$ 304,288		61,544
Interest Rate Swaption
Derivative Instruments:
Realized Gain (Loss)				(21,891)
Unrealized Gain (Loss)				17,083
Amount of Gain/(Loss) Recognized in Net Gains (Losses) on Trading Assets				$ (4,808)

[1] Includes options on TBA securities.