Fair Value Measurements (Details) (USD $) In Thousands, unless otherwise specified			6 Months Ended
	Jun. 30, 2012	Dec. 31, 2011	Jun. 30, 2012 Minimum	Jun. 30, 2012 Maximum	Jun. 30, 2012 Fair value on a recurring basis Fair Value	Dec. 31, 2011 Fair value on a recurring basis Fair Value	Jun. 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)	Jun. 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)	Jun. 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%	58.00%
Assets
Derivative instruments, at fair value	$ 467,587	$ 512,161			$ 467,587	$ 512,161	$ 1,688	$ 3,612	$ 335,352	$ 365,238	$ 130,547	$ 143,311
Total assets					467,587	512,161	1,688	3,612	335,352	365,238	130,547	143,311
Liabilities
Derivative instruments, at fair value					109,214	123,306	1,534	2,727	106,569	120,528	1,111	51
Total liabilities					$ 109,214	$ 123,306	$ 1,534	$ 2,727	$ 106,569	$ 120,528	$ 1,111	$ 51