Fair Value Measurements and Disclosures (Details) (USD $)	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2013 Fair Value Significant Other Observable Inputs (Level 2)	Dec. 31, 2012 Fair Value Significant Other Observable Inputs (Level 2)	Sep. 30, 2013 Fair Value Significant Unobservable Inputs (Level 3)	Dec. 31, 2012 Fair Value Significant Unobservable Inputs (Level 3)	Sep. 30, 2013 Fair Value Total	Dec. 31, 2012 Fair Value Total	Sep. 30, 2013 3.25% Convertible Senior Notes due 2016 Rate	Sep. 30, 2013 7.75% Senior Notes due 2022 Rate
Assets and Liabilities at Fair Value
Document Period End Date	Sep. 30, 2013
Commodity-based derivative - assets		$ 12,328,000	$ 42,788,000	$ 3,060,000	$ 15,734,000	$ 15,388,000	$ 58,522,000
Basis protection derivative - assets		182,000	387,000	12,000	16,000	194,000	403,000
Total assets		12,510,000	43,175,000	3,072,000	15,750,000	15,582,000	58,925,000
Commodity-based derivative - liabilities		17,293,000	9,839,000	2,340,000	2,081,000	19,633,000	11,920,000
Basis protection derivative - liabilities		1,563,000	16,656,000	0	0	1,563,000	16,656,000
Total liabilities		18,856,000	26,495,000	2,340,000	2,081,000	21,196,000	28,576,000
Net asset (fair value)		(6,346,000)	16,680,000	732,000	13,669,000	(5,614,000)	30,349,000
Debt Instrument, Interest Rate, Stated Percentage								3.25%	7.75%
3.25% convertible senior notes fair value								176,400,000
3.25% convertible senior notes fair value as a percentage of par								153.40%
7.75% senior notes fair value									$ 528,800,000
7.75% senior notes fair value as percentage of par									105.70%