Offsetting Assets and Liabilities (Details) (USD $)	Sep. 30, 2013	Dec. 31, 2012
Assets
Gross Amounts of Recognized Assets	$ 9,513,000	$ 9,227,000
Net Amounts of Assets Presented in the Statement of Financial Position	9,513,000	9,227,000
Gross Amounts Not Offset in the Statement of Financial Position
Financial Instruments	6,269,000	4,335,000
Cash Collateral Received	2,080,000	2,989,000
Net Amount	1,164,000	1,903,000
Liabilities
Gross Amounts of Recognized Liabilities	1,046,502,000	1,333,582,000
Net Amounts of Assets Presented in the Statement of Financial Position	1,046,502,000	1,333,582,000
Gross Amounts Not Offset in the Statement of Financial Position
Financial Instruments	1,020,519,000	1,310,147,000
Cash Collateral Pledged	2,239,000
Net Amount	23,744,000	23,435,000
Fair value of assets pledged against repurchase agreements	2,115,500,000	2,502,900,000
Derivatives
Assets
Gross Amounts of Recognized Assets	9,513,000	9,227,000
Net Amounts of Assets Presented in the Statement of Financial Position	9,513,000	9,227,000
Gross Amounts Not Offset in the Statement of Financial Position
Financial Instruments	6,269,000	4,335,000
Cash Collateral Received	2,080,000	2,989,000
Net Amount	1,164,000	1,903,000
Liabilities
Gross Amounts of Recognized Liabilities	32,252,000	27,770,000
Net Amounts of Assets Presented in the Statement of Financial Position	32,252,000	27,770,000
Gross Amounts Not Offset in the Statement of Financial Position
Financial Instruments	6,269,000	4,335,000
Cash Collateral Pledged	2,239,000
Net Amount	23,744,000	23,435,000
Secured financing facilities (1)
Liabilities
Gross Amounts of Recognized Liabilities	1,014,250,000	1,305,812,000
Net Amounts of Assets Presented in the Statement of Financial Position	1,014,250,000	1,305,812,000
Gross Amounts Not Offset in the Statement of Financial Position
Financial Instruments	$ 1,014,250,000	$ 1,305,812,000