Derivative Commodity Instruments - Derivative Instruments Change in Fair Value (Details) (Derivative Commodity Instruments, USD $)	3 Months Ended				9 Months Ended
	Sep. 30, 2013		Sep. 30, 2012		Sep. 30, 2013	Sep. 30, 2012
Derivative Commodity Instruments
Fair Value, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of period	$ 51,131,000		$ 103,456,000		$ 89,019,000	$ 65,801,000
Realized gains	10,852,000		18,737,000		41,952,000	51,858,000
Unrealized losses relating to instruments held at the reporting date	(10,947,000)	[1]	(46,983,000)	[1]	(48,835,000)	(9,328,000)	[1]
Settlements during period	(9,830,000)		(17,929,000)		(40,930,000)	(51,050,000)
Balance at end of period	41,206,000		57,281,000		41,206,000	57,281,000
Mark-to-market gain included in earnings, unrealized	800,000				4,700,000
Mark-to-market loss included in earnings, unrealized			$ 7,900,000			$ 4,500,000

[1] Includes $0.8 million and $4.7 million in mark-to-market gains for the three months and nine months ended September 30, 2013, respectively.