- Change in Fair Value (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Derivative Commodity Instruments
Fair Value, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of period	$ 1,378,000	$ 26,459,000	$ 18,289,000	$ 89,019,000
Realized gains	785,000	6,183,000	(2,158,000)	32,368,000
Unrealized losses relating to instruments held at the reporting date	3,800,000	23,404,000	(13,111,000)	(39,156,000)
Settlements during period	(756,000)	(4,915,000)	2,187,000	(31,100,000)
Balance at end of period	5,207,000	51,131,000	5,207,000	51,131,000
Mark-to-market gain included in earnings, unrealized	5,200,000		4,200,000
Mark-to-market loss included in earnings, unrealized		8,500,000		2,700,000
Level 3
Fair Value, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Impact on fair value of a 10 percent increase or decrease in commodity prices	23,000,000		23,000,000
Impact on results of operations of a 10 percent increase or decrease in commodity prices			$ 23,000,000