Stock-Based Compensation Plans - Additional Information (Detail) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012 Plans	Dec. 31, 2011	Dec. 31, 2010
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period									3 years
Expiration term of stock options									5 years
Operating expense	$ 30,214,000	$ 29,628,000	$ 27,041,000	$ 29,022,000	$ 25,918,000	$ 29,828,000	$ 28,509,000	$ 28,717,000	$ 115,905,000	$ 112,972,000	$ 96,274,000
Exploration expense	18,022,000	14,752,000	15,517,000	21,516,000	24,982,000	17,606,000	11,592,000	27,187,000	69,807,000	81,367,000	60,506,000
General and administrative expense	46,582,000	44,497,000	44,005,000	38,729,000	42,205,000	35,907,000	39,120,000	33,959,000	173,813,000	151,191,000	140,571,000
Stock-based compensation expense									4,049,000	4,108,000	4,209,000
Brokered natural gas and marketing	4,994,000	4,887,000	6,491,000	4,062,000	3,151,000	3,333,000	2,981,000	2,521,000	20,434,000	11,986,000	9,761,000
Termination costs											8,452,000
Cash received upon exercise of stock options/SARs awards									2,100,000
Tax benefit of stock compensation										11,676,000
Active equity-based stock plans									2
Number of stock option granted									754,471	843,485	1,394,136
Number of SARs outstanding	3,400,000								3,400,000
Total intrinsic value of stock options and SARs exercised									61,000,000	62,500,000	50,600,000
Aggregate intrinsic value of the awards outstanding	40,400,000								40,400,000
Aggregate intrinsic value of the awards exercisable	27,000,000								27,000,000
Weighted average remaining contractual life of awards exercisable									1 year 7 months 13 days
Maximum percentage of employers contribution	75.00%								75.00%
Maximum percentage of employers contribution in cash									6.00%
Employer contribution									4,000,000	3,300,000
Minimum period of service for contribution to fully vest									2 years
Non-cash, Mark-to-market income (loss) related to deferred compensation plan									(7,200,000)	(43,200,000)	10,200,000
Shares held in rabbi trust total	2,700,000				2,800,000				2,700,000	2,800,000
Vested shares held in rabbi trust	2,300,000				2,300,000				2,300,000	2,300,000
Stock Option [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of stock option granted									32,000
Number of stock option outstanding	3,400,000								3,400,000
Stock Appreciation Rights (SARs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of fully vested awards and awards expected to vest	3,300,000								3,300,000
Weighted average exercise price of fully vested awards and awards expected to vest in years									2 years 5 months 5 days
Weighted average exercise price of fully vested awards and awards expected to vest	$ 52.45								$ 52.45
Aggregate intrinsic value of fully vested awards and awards expected to vest	39,000,000								39,000,000
Unrecognized compensation related to Awards	16,700,000								16,700,000
Weighted average period									1 year 9 months 18 days
Share Based Compensation Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Operating expense									2,400,000	2,000,000	2,000,000
Exploration expense									4,100,000	4,100,000	4,200,000
General and administrative expense									44,500,000	36,200,000	34,200,000
Stock-based compensation expense									52,800,000	43,800,000	44,400,000
Brokered natural gas and marketing									1,800,000	1,500,000	1,200,000
Termination costs											2,800,000
2005 Equity Based Compensation Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares issued under the plans									5,600,000
Maximum number of common stock authorized to be issued under the Plans	15,000,000								15,000,000
1999 Stock Option Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares issued under the plans									2,200,000
Maximum number of common stock authorized to be issued under the Plans	7,200,000								7,200,000
Director Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum number of common stock authorized to be issued under the Plans	450,000								450,000
Liability Awards [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense									21,500,000	19,100,000	20,500,000
Unrecognized compensation related to Awards	21,600,000								21,600,000
Weighted average period									1 year 10 months 24 days
Shares granted to employees									366,300	334,000	392,000
Vesting period of shares granted to employees									Three-year	Three-year	Three-year
Shares granted to employees and directors									381,000	352,000	413,000
Restricted stock granted average price									$ 64.06	$ 51.17	$ 45.83
Shares granted to directors									14,700	18,000	21,000
Proceeds received from sale of stock held in deferred compensation plan									26,600,000
Equity Awards [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense									11,800,000	4,200,000
Unrecognized compensation related to Awards	$ 15,400,000								$ 15,400,000
Weighted average period									2 years
Shares granted to employees									364,000	331,000
Vesting period of shares granted to employees									Three-year