Fair Value Measurements (Details) (USD $) In Thousands, unless otherwise specified			9 Months Ended
	Sep. 30, 2012	Dec. 31, 2011	Sep. 30, 2012 Minimum	Sep. 30, 2012 Maximum	Sep. 30, 2012 Fair value on a recurring basis Fair Value	Dec. 31, 2011 Fair value on a recurring basis Fair Value	Sep. 30, 2012 Fair value on a recurring basis Quoted prices in active markets for identical assets (Level 1)	Dec. 31, 2011 Fair value on a recurring basis Quoted prices in active markets for identical assets (Level 1)	Sep. 30, 2012 Fair value on a recurring basis Significant other observable inputs (Level 2)	Dec. 31, 2011 Fair value on a recurring basis Significant other observable inputs (Level 2)	Sep. 30, 2012 Fair value on a recurring basis Significant unobservable inputs (Level 3)	Dec. 31, 2011 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)			25.00%	54.00%
Assets
Derivative instruments, at fair value	$ 319,203	$ 512,161			$ 319,203	$ 512,161	$ 2,153	$ 3,612	$ 209,506	$ 365,238	$ 107,544	$ 143,311
Total assets					319,203	512,161	2,153	3,612	209,506	365,238	107,544	143,311
Liabilities
Derivative instruments, at fair value					125,452	123,306	2,529	2,727	112,350	120,528	10,573	51
Total liabilities					$ 125,452	$ 123,306	$ 2,529	$ 2,727	$ 112,350	$ 120,528	$ 10,573	$ 51