Fair Value Measurements and Disclosures (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Fair Value | Significant Other Observable Inputs (Level 2)
Assets and Liabilities at Fair Value
Commodity-based derivative - assets	$ 197,331	$ 237,939
Basis protection derivative - assets	0	19
Total assets	197,331	237,958
Commodity-based derivative - liabilities	482	742
Basis protection derivative - liabilities	2,486	25
Total liabilities	2,968	767
Net asset (fair value)	194,363	237,191
Fair Value | Significant Unobservable Inputs (Level 3)
Assets and Liabilities at Fair Value
Commodity-based derivative - assets	83,862	62,356
Basis protection derivative - assets	0	0
Total assets	83,862	62,356
Commodity-based derivative - liabilities	0	0
Basis protection derivative - liabilities	0	0
Total liabilities	0	0
Net asset (fair value)	83,862	62,356
Fair Value | Total
Assets and Liabilities at Fair Value
Commodity-based derivative - assets	281,193	300,295
Basis protection derivative - assets	0	19
Total assets	281,193	300,314
Commodity-based derivative - liabilities	482	742
Basis protection derivative - liabilities	2,486	25
Total liabilities	2,968	767
Net asset (fair value)	$ 278,225	$ 299,547
3.25% Convertible Senior Notes due 2016
Assets and Liabilities at Fair Value
Debt Instrument, Interest Rate, Stated Percentage	3.25%
3.25% convertible senior notes fair value	$ 161,400
3.25% convertible senior notes fair value as a percentage of par	140.30%
7.75% Senior Notes due 2022
Assets and Liabilities at Fair Value
Debt Instrument, Interest Rate, Stated Percentage	7.75%
7.75% senior notes fair value	$ 496,300
7.75% senior notes fair value as percentage of par	99.30%