Fair Value Measurements (Summary Assets and Liabilities Measured at Fair Value on Recurring Basis) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Significant Observable Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	$ 65,538	$ 287,993
Derivative liabilities	(8,601)
Total	56,937	287,993
Significant Unobservable Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	12,965	4,151
Derivative liabilities	(31,889)	(4,266)
Total	(18,924)	(115)
Fair Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	78,503	292,144
Derivative liabilities	(40,490)	(4,266)
Total	$ 38,013	$ 287,878