Stock-based Compensation Plans - Additional Information (Detail) (USD $)	3 Months Ended		9 Months Ended			12 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012 OptionPlan		Sep. 30, 2011	Dec. 31, 2011
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period			3 years
Expiration term of stock options			5 years
Operating expense	$ 29,628,000	$ 29,828,000	$ 85,691,000		$ 87,054,000
Exploration expense	14,752,000	17,606,000	51,785,000		56,385,000
General and administrative expense	44,497,000	35,907,000	127,231,000		108,986,000
Stock-Based Compensation Expense	12,200,000	10,200,000	36,800,000		33,200,000
Active equity based stock plans			2
Number of Shares Outstanding Under the Plans	3,603,570		3,603,570			4,558,609
Number of stock option outstanding	32,450		32,450
Number of SARs outstanding	3,600,000		3,600,000
Stock-based compensation expense			3,047,000	[1]		4,108,000	[1]
Restricted Stock granted average price			$ 21.32
Non-cash, Mark-to-market income (loss) related to deferred compensation plan	20,100,000	8,700,000	21,600,000		33,600,000
Shares held in rabbi trust total	2,800,000		2,800,000			2,800,000
Vested shares held in rabbi trust	2,300,000		2,300,000			2,300,000
Share Based Compensation Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Operating expense	(598,000)	(463,000)	(1,600,000)		(1,400,000)
Exploration expense	(1,100,000)	(902,000)	(3,000,000)		(3,200,000)
General and administrative expense	(10,100,000)	(8,500,000)	(30,800,000)		(27,500,000)
Equity Awards [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares granted to employees			360,000		329,000
Vesting period of shares granted to employees			3
Stock-based compensation expense			8,200,000		2,900,000
Restricted Stock granted average price			$ 63.37
Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted Stock granted average price			$ 63.37		$ 49.47
Liability Awards [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares granted to employees			356,000		334,200
Stock-based compensation expense			$ 15,200,000		$ 14,600,000
Restricted Stock granted average price			$ 63.90
Restricted Stock granted to employees average price			$ 63.88		$ 51.11
Restricted Stock granted to directors average price			$ 64.35		$ 52.35
Shares granted to directors			14,700		15,500

[1] Includes cost incurred whether capitalized or expensed.