Fair Value Measurements (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Assets
Derivative instruments, at fair value	$ 512,161	$ 225,339
Liabilities
Derivative instruments, at fair value		106,721
Fair value on a recurring basis | Fair Value
Assets
Investments, available-for-sale		28,968
Derivative instruments, at fair value	512,161	225,339
Total assets	512,161	254,307
Liabilities
Derivative instruments, at fair value	123,306	106,721
Total liabilities	123,306	106,721
Fair value on a recurring basis | Quoted prices in active markets for identical assets (Level 1)
Assets
Investments, available-for-sale		28,968
Derivative instruments, at fair value	3,612	8,968
Total assets	3,612	37,936
Liabilities
Derivative instruments, at fair value	2,727	7,627
Total liabilities	2,727	7,627
Fair value on a recurring basis | Significant other observable inputs (Level 2)
Assets
Derivative instruments, at fair value	365,238	99,489
Total assets	365,238	99,489
Liabilities
Derivative instruments, at fair value	120,528	98,884
Total liabilities	120,528	98,884
Fair value on a recurring basis | Significant unobservable inputs (Level 3)
Assets
Derivative instruments, at fair value	143,311	116,882
Total assets	143,311	116,882
Liabilities
Derivative instruments, at fair value	51	210
Total liabilities	$ 51	$ 210