Fair Value Measurements (Details) (USD $) In Thousands	Sep. 30, 2011	Dec. 31, 2010
Assets
Derivative instruments, at fair value	$ 313,537	$ 225,339
Liabilities
Derivative instruments, at fair value	108,926	106,721
Fair value on a recurring basis | Fair Value
Assets
Derivative instruments, at fair value	313,537
Total assets	313,537
Liabilities
Derivative instruments, at fair value	108,926
Total liabilities	108,926
Fair value on a recurring basis | Quoted prices in active markets for identical assets (Level 1)
Assets
Derivative instruments, at fair value	7,014
Total assets	7,014
Liabilities
Derivative instruments, at fair value	8,271
Total liabilities	8,271
Fair value on a recurring basis | Significant other observable inputs (Level 2)
Assets
Derivative instruments, at fair value	193,439
Total assets	193,439
Liabilities
Derivative instruments, at fair value	100,655
Total liabilities	100,655
Fair value on a recurring basis | Significant unobservable inputs (Level 3)
Assets
Derivative instruments, at fair value	113,084
Total assets	$ 113,084