- Change in Fair Value (Details) (Derivative Commodity Instruments, USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Derivative Commodity Instruments
Fair Value, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Mark-to-market gain included in earnings, unrealized	$ 12,200,000		$ 9,600,000
Mark-to-market loss included in earnings, unrealized		800,000		4,700,000
Balance at beginning of period	5,207,000	51,131,000	18,289,000	89,019,000
Realized gains	10,769,000	10,852,000	8,581,000	41,952,000
Unrealized gain (losses) relating to instruments held at the reporting date	9,348,000	(10,947,000)	(3,734,000)	(48,835,000)
Settlements during period	(10,438,000)	(9,830,000)	(8,250,000)	(40,930,000)
Balance at end of period	$ 14,886,000	$ 41,206,000	$ 14,886,000	$ 41,206,000