Organization and Summary of Significant Accounting Policies (Details) (USD $)	3 Months Ended		12 Months Ended
	Sep. 30, 2012	Jun. 30, 2012	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Asset Retirement Obligation, Period Increase (Decrease)			$ 39,000,000
Natural Gas Storage Revenue			900,000	900,000	2,500,000
Outstanding checks included in accounts payable			12,100,000	47,700,000
Impairment Of Oil And Gas Properties Net Of Tax	185,700,000	496,400,000	510,400,000
Cash flow hedges impact on ceiling value			257,600,000
Cash paid during the year for interest, net of amounts capitalized			17,311,000	19,159,000	24,049,000
Cash paid during the year for income taxes			818,000	4,198,000	14,368,000
Increase (decrease) in noncash property additions			(26,240,000)	30,389,000	46,588,000
Common Stock in Treasury, Shares			64,715	98,889
Over Production [Member]
Gas imbalance volume amount in Bcf			8.4	6.1
Gas imbalance asset (liability)			28,400,000	22,500,000
Under Production [Member]
Gas imbalance volume amount in Bcf			9.4	6.4
Gas imbalance asset (liability)			29,900,000	22,900,000
Henry Hub Natural Gas [Member]
Full cost ceiling test, Price			2.76	4.12	4.38
West Texas Intermediate Oil [Member]
Full cost ceiling test, Price			91.21	92.71	75.96
Stock Options [Member]
Incremental Common Shares Attributable to Share-based Payment Arrangements				3,577,104	4,753,530
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, Exercisable Options, Weighted Average Exercise Price				$ 11.78	$ 9.42
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount			1,716,109	881,254	548,160
Restricted Stock [Member]
Incremental Common Shares Attributable to Share-based Payment Arrangements				241,044	700,512
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount			602,429	135,352	39,600
Exploration and Production [Member]
Energy Related Inventory, Gas Stored Underground			5,600,000	7,800,000
Current assets inventory, tubulars & other equipment			22,500,000	38,400,000
Midstream Services [Member]
Other Inventory, Noncurrent			$ 13,800,000	$ 19,500,000