Significant Accounting Policies (Details) (USD $) Share data in Thousands, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012
Significant Accounting Policies Details [Abstract]
Discount Rate Future Net Revenues			10.00%
Earnings Per Share Reconciliation [Abstract]
Net Income (Loss)	$ 116,377,000	$ (1,186,982,000)	$ 132,811,000	$ (1,102,723,000)
Weighted Average Number of Shares Outstanding, Basic	152,948	152,921	152,947	152,761
Incremental Common Shares Attributable to Share-based Payment Arrangements	1,565	0	1,450	0
Weighted Average Number of Shares Outstanding, Diluted	154,513	152,921	154,397	152,761
Earnings Per Share, Basic	$ 0.76	$ (7.76)	$ 0.87	$ (7.22)
Earnings Per Share, Diluted	$ 0.75	$ (7.76)	$ 0.86	$ (7.22)
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	1,332	1,859	2,014	1,886
Ceiling Test Limitation Pretax Oil and Gas Properties					$ 2,900,000,000