Derivative Commodity Instruments - Derivatives Measured at Fair Value on a Recurring Basis (Details) (USD $)							3 Months Ended
	Mar. 31, 2014	Dec. 31, 2013	Mar. 31, 2014 Level 2 Fair Value, Measurements, Recurring		Dec. 31, 2013 Level 2 Fair Value, Measurements, Recurring		Mar. 31, 2014 Level 3	Mar. 31, 2014 Level 3 Fair Value, Measurements, Recurring		Dec. 31, 2013 Level 3 Fair Value, Measurements, Recurring		Mar. 31, 2014 Total Fair Value, Measurements, Recurring	Dec. 31, 2013 Total Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Current assets			$ (6,646,000)	[1]	$ (1,658,000)	[1]		$ 10,003,000	[1]	$ 19,121,000	[1]	$ 3,357,000	$ 17,463,000
Noncurrent assets	2,638,000	5,439,000	1,192,000	[1]	4,383,000	[1]		1,446,000	[1]	1,056,000	[1]	2,638,000	5,439,000
Current liabilities			(42,531,000)	[1]	(28,414,000)	[1]		(10,071,000)	[1]	(1,888,000)	[1]	(52,602,000)	(30,302,000)
Noncurrent liabilities	(1,011,000)	(398,000)	(802,000)	[1]				0	[1]			(802,000)
Net derivative asset (liability)			(48,787,000)	[1]	(25,689,000)	[1]		1,378,000	[1]	18,289,000	[1]	(47,409,000)	(7,400,000)
Impact on fair value of a 10 percent increase or decrease in commodity prices							18,000,000
Impact on results of operations of a 10 percent increase or decrease in commodity prices							$ 18,000,000

[1] Amounts classified in accordance with accounting guidance which permits offsetting fair value amounts recognized for multiple derivative instruments executed with the same counterparty under a master netting arrangement.