Effect of Other Derivative Contracts on the Consolidated Statements of Operations and Comprehensive Income (Loss) (Detail) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Derivative Instruments:
Unrealized Gain (Loss)		$ (177,567)	$ (373,220)	$ (28,383)	$ (1,404,940)
Amount of Gain/(Loss) Recognized in Net Gains (Losses) on Trading Assets		(14,423)	81,880	(14,123)	207,052
U.S. Treasury Futures
Derivative Instruments:
Realized Gain (Loss)		(59,397)	8,314	(58,424)	(122,680)
Unrealized Gain (Loss)		(37,588)	(85,563)	(61,292)	(133,683)
Amount of Gain/(Loss) Recognized in Net Gains (Losses) on Trading Assets		(96,985)	(77,249)	(119,716)	(256,363)
Purchase Commitments
Derivative Instruments:
Unrealized Gain (Loss)		8		272
Amount of Gain/(Loss) Recognized in Net Gains (Losses) on Trading Assets		8		272
TBA Derivatives
Derivative Instruments:
Realized Gain (Loss)	[1]	165,777	98,371	105,463	318,363
Unrealized Gain (Loss)	[1]	(72,844)	60,758	10,237	145,052
Amount of Gain/(Loss) Recognized in Net Gains (Losses) on Trading Assets	[1]	92,933	$ 159,129	115,700	$ 463,415
Interest Rate Swaption
Derivative Instruments:
Unrealized Gain (Loss)		(10,438)		(10,438)
Amount of Gain/(Loss) Recognized in Net Gains (Losses) on Trading Assets		(10,438)		(10,438)
Credit Derivatives
Derivative Instruments:
Realized Gain (Loss)		136		136
Unrealized Gain (Loss)		(77)		(77)
Amount of Gain/(Loss) Recognized in Net Gains (Losses) on Trading Assets		$ 59		$ 59

[1] Includes options on TBA contracts.