Derivative Instruments (Derivative Instruments Offsetting Balance Sheet) (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013		Dec. 31, 2012
Derivative Assets [Abstract]
Gross Amounts Recognized	$ 109,819		$ 190,634
Gross Amounts Offset in the Balance Sheet	(22,398)		(22,426)
Net Amounts Presented in the Balance Sheet	87,421		168,208
Derivative Asset, Fair Value of Collateral	(10,624)	[1]	(22,458)	[1]
Gross Amounts Not Offset in the Balance Sheet - Cash Collateral Pledged/Received	(64,027)	[2]	(136,414)	[2]
Net Amount	12,770		9,336
Derivative Liabilities [Abstract]
Gross Amounts Recognized	35,546		54,078
Gross Amounts Offset in the Balance Sheet	(22,398)		(22,426)
Net Amounts Presented in the Balance Sheet	13,148		31,652
Gross Amounts Not Offset in the Balance Sheet - Financial Instruments	(15,238)	[3]	(1,565)	[3]
Gross Amounts Not Offset in the Balance Sheet - Cash Collateral Pledged/Received	(8,150)	[4]	(27,867)	[4]
Net Amount	$ (10,240)		$ 2,220

[1] Consists of U.S. Treasury securities.

[2] Included in cash and cash equivalents, with obligation to return cash collateral recorded in other liabilities.

[3] Included in other invested assets, primarily consists of U.S. Treasury securities.

[4] Consists of receivable from counterparty, included in other assets.