Derivative Instruments Offsetting Balance Sheet (Details) (Derivative [Member], USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Offsetting Assets [Member]
Offsetting of Financial Assets and Liabilities [Line Items]
Gross Amounts of Recognized Financial Instruments	$ 105,458	$ 174,087
Gross Amounts Offset in Statement of Financial Position	(75,568)	(42,352)
Net Amounts of Assets Presented in Statement of Financial Position	29,890	131,735
Financial Instruments	0	0
Net Amount	29,890	131,735
Offsetting Liabilities [Member]
Offsetting of Financial Assets and Liabilities [Line Items]
Gross Amounts of Recognized Financial Instruments	75,568	42,352
Gross Amounts Offset in Statement of Financial Position	(75,568)	(42,352)
Net Amounts of Assets Presented in Statement of Financial Position	0	0
Financial Instruments	0	0
Net Amount	$ 0	$ 0