Offsetting Assets and Liabilities (Details) (USD $)	9 Months Ended
	Sep. 30, 2013 counterparty		Dec. 31, 2012
Derivative hedging instruments, at fair value
Gross Amounts of Recognized Assets	$ 4,629,000		$ 203,000
Gross Amounts offset in the Consolidated Balance Sheets	0		0
Net Amounts of Assets Presented in the Consolidated Balance Sheet	4,629,000		203,000
Derivative Fair Value of Derivative Asset Gross Amounts Not Offset [Abstract]
Financial Instruments	(4,629,000)		(203,000)
Cash Collateral Received	0		0
Net Amount	0		0
Derivative hedging instruments, at fair value
Gross Amounts of Recognized Liabilities	65,253,000		63,034,000
Repurchase agreements
Gross Amounts of Recognized Liabilities	8,568,171,000	[1]	8,752,472,000	[1]
Gross Amounts Offset in the Consolidated Balance Sheet	0	[1]	0	[1]
Net Amounts of Liabilities Presented in the Consolidated Balance Sheet	8,568,171,000	[1]	8,752,472,000	[1]
Gross Amounts Not Offset in the Consolidated Balance Sheet
Financial Instruments	(8,568,171,000)	[1],[2]	(8,752,472,000)	[1],[2]
Cash Collateral Pledged	0	[1],[2]	0	[1],[2]
Net Amount	0	[1]	0	[1]
Total
Gross Amounts of Recognized Liabilities	8,624,700,000		8,815,506,000
Gross Amounts Offset in the Consolidated Balance Sheet	0		0
Net Amounts of Liabilities Presented in the Consolidated Balance Sheet	8,624,700,000		8,815,506,000
Gross Amounts Not Offset in the Consolidated Balance Sheet
Financial Instruments	8,565,425,000	[2]	8,810,490,000	[2]
Cash Collateral Pledged	59,275,000	[2]	5,016,000	[2]
Net Amount	0		0
Additional disclosures
Fair value of securities pledged against the swaps	67,300,000		68,900,000
Fair value of securities pledged against the repurchase agreements	10,189,000,000		10,550,000,000
Number of repurchase agreement counterparties with unconditional right of setoff	1
Hedging instrument
Derivative hedging instruments, at fair value
Gross Amounts of Recognized Assets	4,629,000		203,000
Gross Amounts offset in the Consolidated Balance Sheets	0		0
Net Amounts of Assets Presented in the Consolidated Balance Sheet	4,629,000		203,000
Derivative Fair Value of Derivative Asset Gross Amounts Not Offset [Abstract]
Financial Instruments	(4,629,000)		(203,000)
Cash Collateral Received	0		0
Net Amount	0		0
Derivative hedging instruments, at fair value
Gross Amounts of Recognized Liabilities	56,529,000	[3]	63,034,000	[3]
Gross Amounts Offset in the Consolidated Balance Sheet	0	[3]	0	[3]
Net Amounts of Liabilities Presented in the Consolidated Balance Sheet	56,529,000	[3]	63,034,000	[3]
Gross Amounts Not Offset in the Consolidated Balance Sheet
Financial Instruments	2,746,000	[2],[3]	(58,018,000)	[2],[3]
Cash Collateral Pledged	(59,275,000)		(5,016,000)	[2],[3]
Net Amount	$ 0	[3]	$ 0	[3]

[1] The fair value of securities pledged against the Company’s repurchase agreements was $10.189 billion and $10.550 billion at September 30, 2013 and December 31, 2012, respectively.

[2] Amounts disclosed in the Financial Instruments column of the table above represents collateral pledged that is available to be offset against liability balances associated with repurchase agreement and derivative transactions. Amounts disclosed in the Cash Collateral Pledged column of the table above represents amounts pledged as collateral against derivative transactions.

[3] The fair value of securities pledged against the Company’s Swaps was $67.3 million and $68.9 million at September 30, 2013 and December 31, 2012, respectively.