Liabilities and Assets Subject to Netting Arrangements (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014		Dec. 31, 2013
Derivative Assets Derivative Liabilities And Repurchase Agreements Subject To Netting Agreements [Line Items]
Gross Amounts of Recognized Asset	$ 662,095		$ 22,551
Gross Asset Amounts Offset in the Balance Sheets
Derivative instruments at fair value	662,095		22,551
Gross Asset Amounts Not Offset Financial instruments	(662,095)	[1]	(22,551)	[1]
Gross Asset Amounts Not Offset Cash Collateral Received		[1]		[1]
Gross Asset Amounts Not Offset Net Amounts
Gross Amounts of Recognized Liabilities	7,221,288		7,635,914
Gross Liabilities Offset in the Balance Sheets
Net Amounts of Liabilities Presented in the Balance Sheets	7,221,288		7,635,914
Gross Liabilities Not Offset Financial instruments	(7,221,288)	[1]	(7,635,914)	[1]
Gross Liabilities Not Offset Cash Collateral Received		[1]		[1]
Gross Liabilities Not Offset Net Amounts	0
Derivative Financial Instruments, Assets
Derivative Assets Derivative Liabilities And Repurchase Agreements Subject To Netting Agreements [Line Items]
Gross Amounts of Recognized Asset	662,095	[2]	22,551	[2]
Gross Asset Amounts Offset in the Balance Sheets		[2]		[2]
Derivative instruments at fair value	662,095	[2]	22,551	[2]
Gross Asset Amounts Not Offset Financial instruments	(662,095)	[1],[2]	(22,551)	[1],[2]
Gross Asset Amounts Not Offset Cash Collateral Received		[1],[2]		[1],[2]
Gross Asset Amounts Not Offset Net Amounts		[2]		[2]
Repurchase Agreements
Derivative Assets Derivative Liabilities And Repurchase Agreements Subject To Netting Agreements [Line Items]
Gross Amounts of Recognized Liabilities	6,550,000	[3]	7,580,000	[3]
Gross Liabilities Offset in the Balance Sheets		[3]		[3]
Net Amounts of Liabilities Presented in the Balance Sheets	6,550,000	[3]	7,580,000	[3]
Gross Liabilities Not Offset Financial instruments	(6,550,000)	[1],[3]	(7,580,000)	[1],[3]
Gross Liabilities Not Offset Cash Collateral Received		[1],[3]		[1],[3]
Gross Liabilities Not Offset Net Amounts	0	[3]		[3]
Derivative Financial Instruments, Liabilities
Derivative Assets Derivative Liabilities And Repurchase Agreements Subject To Netting Agreements [Line Items]
Gross Amounts of Recognized Liabilities	671,288	[2]	55,914
Gross Liabilities Offset in the Balance Sheets		[2]
Net Amounts of Liabilities Presented in the Balance Sheets	671,288	[2]	55,914
Gross Liabilities Not Offset Financial instruments	(671,288)	[1],[2]	(55,914)	[1]
Gross Liabilities Not Offset Cash Collateral Received		[1],[2]		[1]
Gross Liabilities Not Offset Net Amounts	$ 0	[2]

[1] Amounts presented are limited to collateral pledged sufficient to reduce the related net amount to zero in accordance with ASU No. 2011-11, as amended by ASU No. 2013-01.

[2] At September 30, 2014, we had pledged approximately $26.1 million in Agency MBS as collateral and paid another approximately $7.2 million on swap margin calls on our swap derivatives. At December 31, 2013, we had pledged approximately $84.2 million in Agency MBS as collateral and paid another approximately $7.1 million on swap margin calls on our swap derivatives

[3] At September 30, 2014, we had pledged approximately $6.89 billion in Agency MBS as collateral on our repurchase agreements. At December 31, 2013, we had pledged approximately $7.98 billion in Agency MBS as collateral on our repurchase agreements