Fair Value Measurements (Details) (USD $) In Thousands, unless otherwise specified			6 Months Ended
	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 2013 Minimum	Jun. 30, 2013 Maximum	Jun. 30, 2013 Fair value on a recurring basis Fair Value	Dec. 31, 2012 Fair value on a recurring basis Fair Value	Jun. 30, 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)	Jun. 30, 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)	Jun. 30, 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)			20.00%	32.00%
Assets
Derivative instruments, at fair value	$ 230,139	$ 304,237			$ 230,139	$ 304,237	$ 907	$ 1,228	$ 153,342	$ 204,592	$ 75,890	$ 98,417
Liabilities
Derivative instruments, at fair value	$ 36,471	$ 75,562			$ 36,471	$ 75,562	$ 1,180	$ 1,609	$ 34,089	$ 66,250	$ 1,202	$ 7,703