Fair Value Measurements (Details) (USD $) In Thousands, unless otherwise specified			3 Months Ended
	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2013 Minimum	Mar. 31, 2013 Maximum	Mar. 31, 2013 Fair value on a recurring basis Fair Value	Dec. 31, 2012 Fair value on a recurring basis Fair Value	Mar. 31, 2013 Fair value on a recurring basis Quoted prices in active markets for identical assets (Level 1)	Dec. 31, 2012 Fair value on a recurring basis Quoted prices in active markets for identical assets (Level 1)	Mar. 31, 2013 Fair value on a recurring basis Significant other observable inputs (Level 2)	Dec. 31, 2012 Fair value on a recurring basis Significant other observable inputs (Level 2)	Mar. 31, 2013 Fair value on a recurring basis Significant unobservable inputs (Level 3)	Dec. 31, 2012 Fair value on a recurring basis Significant unobservable inputs (Level 3)
Fair value assets and liabilities measured on a recurring basis
Range of derived market volatilities (as a percent)			22.00%	31.00%
Assets
Derivative instruments, at fair value	$ 183,727	$ 304,237			$ 183,727	$ 304,237	$ 1,588	$ 1,228	$ 105,620	$ 204,592	$ 76,519	$ 98,417
Liabilities
Derivative instruments, at fair value	$ 92,107	$ 75,562			$ 92,107	$ 75,562	$ 2,382	$ 1,609	$ 83,071	$ 66,250	$ 6,654	$ 7,703