Significant Accounting Policies (Details) (USD $) Share data in Thousands, except Per Share data, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Significant Accounting Policies Details [Abstract]
Discount Rate Future Net Revenues	10.00%
Earnings Per Share Reconciliation [Abstract]
Net Income (Loss)	$ 16,433,000	$ 84,262,000
Weighted Average Number of Shares Outstanding, Basic	152,947	152,601
Incremental Common Shares Attributable to Share-based Payment Arrangements	1,523	917
Weighted Average Number of Shares Outstanding, Diluted	154,470	153,518
Earnings Per Share, Basic	$ 0.11	$ 0.55
Earnings Per Share, Diluted	$ 0.11	$ 0.55
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	1,331	1,864
Ceiling Test Limitation Pretax Oil and Gas Properties			$ 2,900,000,000