Derivative Commodity Instruments - Derivative Instruments Change in Fair Value (Details) (Derivative Commodity Instruments, USD $)	3 Months Ended
	Mar. 31, 2013		Mar. 31, 2012
Derivative Commodity Instruments
Fair Value, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of period	$ 89,019,000		$ 65,801,000
Realized gains	27,107,000		13,181,000
Unrealized gains (losses) relating to instruments held at the reporting date	(63,482,000)	[1]	39,767,000	[1]
Settlements during period	(26,185,000)		(13,826,000)
Balance at end of period	26,459,000		104,923,000
Mark-to-market loss included in earnings, unrealized	12,400,000
Mark-to-market gain included in earnings, unrealized			$ 3,800,000

[1] Includes $12.4 million in mark-to-market losses and $3.8 million in mark-to-market gains for the three months ended March 31, 2013 and 2012.