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, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Derivative Instruments:
Unrealized Gain (Loss)		$ 56,854	$ 256,462	$ 28,471	$ (1,148,478)
Amount of Gain/(Loss) Recognized in Net Gains (Losses) on Trading Assets		154,208	162,981	140,086	370,034
U.S. Treasury Futures
Derivative Instruments:
Realized Gain (Loss)		(70,054)	(47,035)	(128,478)	(169,716)
Unrealized Gain (Loss)		92,784	110,218	31,492	(23,465)
Amount of Gain/(Loss) Recognized in Net Gains (Losses) on Trading Assets		22,730	63,183	(96,986)	(193,181)
Purchase Commitments
Derivative Instruments:
Unrealized Gain (Loss)		(108)	116	165	116
Amount of Gain/(Loss) Recognized in Net Gains (Losses) on Trading Assets		(108)	116	165	116
TBA Derivatives
Derivative Instruments:
Realized Gain (Loss)	[1]	110,067	174,086	215,529	492,450
Unrealized Gain (Loss)	[1]	29,728	(78,677)	39,964	66,376
Amount of Gain/(Loss) Recognized in Net Gains (Losses) on Trading Assets	[1]	139,795	95,409	255,493	558,826
Interest Rate Swaption
Derivative Instruments:
Realized Gain (Loss)			4,180		4,180
Unrealized Gain (Loss)		(9,137)	93	(19,574)	93
Amount of Gain/(Loss) Recognized in Net Gains (Losses) on Trading Assets		(9,137)	$ 4,273	(19,574)	$ 4,273
Credit Derivatives
Derivative Instruments:
Realized Gain (Loss)		495		632
Unrealized Gain (Loss)		433		356
Amount of Gain/(Loss) Recognized in Net Gains (Losses) on Trading Assets		$ 928		$ 988

[1] Includes options on TBA contracts.