Offsetting Assets and Liabilities (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013		Dec. 31, 2012
Offsetting of Assets and Liabilities [Line Items]
Reverse Repurchase Agreements	$ (965,272)		$ (905,718)
Financial derivatives-liabilities at fair value-	(18,490)		(15,212)
Financial derivatives-assets at fair value-	36,250		48,504
Derivative Asset, Fair Value, Net	17,760	[1]	33,292	[1]
Fair value	42,614	[1]	13,650	[1]
Net Value, Repurchase and Reverse Repurchase Agreements	(922,658)	[1]	(892,068)	[1]
Securities Collateral on Derivatives	0	[2],[3]	0	[2],[3]
Securities Collateral (Received) Pledged for Repurchase and Reverse Repurchase Agreements	1,111,079	[2],[3]	1,047,944	[2],[3]
Cash collateral (Received) Pledged for Derivatives	(2,620)	[3]	(13,188)	[3]
Cash Collateral (Received) Pledged, Reverse and Reverse Repurchase Agreements	1,730	[3]	4,128	[3]
Net Amount, Derivatives	15,140		20,104
Net Amount, Repurchase and Reverse Repurchase Agreements	190,151		160,004
Financial Derivatives - Assets [Member]
Offsetting of Assets and Liabilities [Line Items]
Financial derivatives-assets at fair value-	36,250	[1],[4]	48,504	[1],[4]
Financial Derivatives - Liabilities [Member]
Offsetting of Assets and Liabilities [Line Items]
Financial derivatives-liabilities at fair value-	(18,490)	[1],[4]	(15,212)	[1],[4]
Securities Sold under Agreements to Repurchase [Member]
Offsetting of Assets and Liabilities [Line Items]
Reverse Repurchase Agreements	$ (956,272)	[1]	$ (905,718)	[1]

[1] In the Company’s Consolidated Statement of Assets, Liabilities, and Equity, all balances associated with the repurchase agreements, reverse repurchase agreements, and derivatives transactions are presented on a gross basis.

[2] Amounts disclosed in the Financial Instruments column of the table represent collateral that is available to be offset against balances associated with repurchase and reverse repurchase agreements.

[3] As collateral is called or posted per counterparty it is generally called or posted across all positions with each respective counterparty.

[4] See Note 2 and Note 3 in Notes to Consolidated Financial Statements.