Derivative Commodity Instruments - Derivative Instruments Change in Fair Value (Details) (Derivative Commodity Instruments, USD $)	3 Months Ended
	Mar. 31, 2014		Mar. 31, 2013
Derivative Commodity Instruments
Fair Value, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of period	$ 18,289,000		$ 89,019,000
Realized gains (losses)	(3,019,000)		27,107,000
Unrealized losses relating to instruments held at the reporting date	(16,835,000)	[1]	(63,482,000)	[1]
Settlements during period	2,943,000		(26,185,000)
Balance at end of period	1,378,000		26,459,000
Mark-to-market gain included in earnings, unrealized	$ 13,200,000		$ 12,400,000

[1] Includes $13.2 million and $12.4 million in mark-to-market losses for the three months ended March 31, 2014 and 2013, respectively.