Derivative Financial Instruments Derivative Financial Instruments (Derivative Asset and Liability Balance Sheet Offsetting) (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013		Dec. 31, 2012		Mar. 31, 2012
Derivative Assets
Gross Amounts Recognized	$ 46,653		$ 47,546		$ 35,027
Less: Amounts offset in the Statements of Financial Condition	0		0		0
Offsetting Derivative Positions	46,653		47,546		35,027
Cash Collateral Posted	0		0		0
Net Credit Exposure	45,130		47,207		34,966
Derivative Liabilities
Gross Amounts Recognized	50,262		53,755		44,884
Less: Amounts offset in the Statements of Financial Condition	0		0		0
Offsetting Derivative Positions	50,262		53,755		44,884
Cash Collateral Posted	(2,445)		(6,605)		(7,055)
Net Credit Exposure	2,933		0		8,415
Derivative Asset, Fair Value, Gross Liability Not Offset in the Statement of Financial Condition	(1,523)		(339)		(61)
Derivative Liability, Fair Value, Gross Asset Not Offset in the Statement of Financial Condition	(1,523)		(339)		(61)
Derivative, Collateral, Obligation to Return Securities	0	[1]	0	[1]	0	[1]
Derivative, Collateral, Right to Reclaim Securities	$ (43,361)	[1]	$ (46,811)	[1]	$ (29,353)	[1]

[1] As of December 31, 2012, the Company posted securities collateral of $49.9 million which resulted in excess collateral with its counterparties. For purposes of this disclosure, the amount of posted collateral is limited to the amount offsetting the derivative liability.