Derivative Instruments Offsetting Balance Sheet (Details) (Derivative [Member], USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Offsetting Assets [Member]
Offsetting of Financial Assets and Liabilities [Line Items]
Gross Amounts of Recognized Financial Instruments	$ 139,739	$ 174,087
Gross Amounts Offset in Statement of Financial Position	(51,033)	(42,352)
Net Amounts of Assets Presented in Statement of Financial Position	88,706	131,735
Financial Instruments	0	0
Net Amount	88,706	131,735
Offsetting Liabilities [Member]
Offsetting of Financial Assets and Liabilities [Line Items]
Gross Amounts of Recognized Financial Instruments	51,033	42,352
Gross Amounts Offset in Statement of Financial Position	(51,033)	(42,352)
Net Amounts of Assets Presented in Statement of Financial Position	0	0
Financial Instruments	0	0
Net Amount	$ 0	$ 0