Stock-Based Compensation Plans - Additional Information (Detail) (USD $)	3 Months Ended			12 Months Ended
	Mar. 31, 2013 Plans		Mar. 31, 2012	Dec. 31, 2012
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting period	3 years
Expiration term of stock options	5 years
Operating expense	$ 30,188,000		$ 29,022,000
Brokered natural gas and marketing expense	22,315,000		4,062,000
Exploration expense	16,780,000		21,516,000
General and administrative	84,058,000		38,729,000
Stock-based compensation expense	1,070,000	[1]		4,049,000	[1]
Active equity-based stock plans	2
Non-cash, Mark-to-market income (loss) related to deferred compensation plan	(42,400,000)		7,800,000
Shares held in rabbi trust total	2,600,000			2,700,000
Vested shares held in rabbi trust	2,200,000			2,300,000
Equity Awards
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	4,300,000		2,100,000
Shares granted to employees	386,000		356,000
Restricted Stock granted average price	$ 71.02		$ 63.37
Vesting period of shares granted to employees	Three-year
Liability Awards
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	4,500,000		4,200,000
Vesting period of shares granted to employees	Three-year		Three-year
Shares granted to employees and directors	125,000		140,000
Restricted stock granted average price	$ 71.40		$ 63.15
Stock Option
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of stock option outstanding	2,593,809
Share Based Compensation Expense
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Operating expense	661,000		357,000
Brokered natural gas and marketing expense	249,000		453,000
Exploration expense	1,100,000		928,000
General and administrative	10,300,000		8,200,000
Stock-based compensation expense	$ 12,300,000		$ 9,900,000

[1] Includes cost incurred whether capitalized or expensed.