Indebtedness - Additional Information (Detail) (USD $)	1 Months Ended	3 Months Ended		9 Months Ended			3 Months Ended	9 Months Ended				3 Months Ended	9 Months Ended	3 Months Ended	9 Months Ended
	Apr. 30, 2012 CommercialBank	Sep. 30, 2012 CommercialBank	Sep. 30, 2011	Sep. 30, 2012 CommercialBank	Sep. 30, 2011	Dec. 31, 2011	Mar. 31, 2012 5.00% Senior Subordinated notes due 2022 [Member]	Sep. 30, 2012 5.00% Senior Subordinated notes due 2022 [Member]	Dec. 31, 2011 5.00% Senior Subordinated notes due 2022 [Member]	Feb. 15, 2020 Subsequent Event [Member] 5.00% Senior Subordinated notes due 2022 [Member]	Feb. 15, 2017 Subsequent Event [Member] 5.00% Senior Subordinated notes due 2022 [Member]	Sep. 30, 2012 Maximum [Member] Ratio	Sep. 30, 2012 Maximum [Member] Ratio	Sep. 30, 2012 Minimum [Member] Ratio	Sep. 30, 2012 Minimum [Member] Ratio
Debt Instrument [Line Items]
Interest costs, capitalized during the period		$ 0	$ 0	$ 0	$ 0
Bank Credit facility, Facility amount		1,750,000,000		1,750,000,000
Bank Credit facility, Borrowing base		2,000,000,000		2,000,000,000
Number of commercial banks included in current bank group		28		28
Percentage holding of commercial banks, Maximum		9.00%		9.00%
Notice period for increase in facility amount				20 days
Number of commercial banks added to bank group	3
Outstanding balance under the bank credit facility		461,000,000		461,000,000		187,000,000
Undrawn letters of credit		62,100,000		62,100,000
Borrowing capacity available under the facility amount		1,200,000,000		1,200,000,000
Maturity date of loan				Feb. 18, 2016
Spread for Alternate Base Rate												1.50%	1.50%	0.50%	0.50%
Spread for LIBOR Rate												2.50%	2.50%	1.50%	1.50%
Weighted average interest rate on the bank credit facility		2.10%		2.20%	2.20%
Annual rate of commitment fee paid on the undrawn balance												0.50%	0.50%	0.375%	0.375%
Commitment fee				0.375%
Interest rate margin on LIBOR loans		1.50%		1.50%
Interest rate margin on base rate loans		0.50%		0.50%
Aggregate principal amount of senior subordinated notes							600,000,000
Net proceeds after underwriting discounts and commissions				600,000,000	500,000,000		589,500,000
Discount on Senior Subordinated notes							10,500,000
Maximum redemption price of notes as percentage of principal amount								105.00%		100.00%	102.50%
Maximum percentage of original aggregate amount of notes that can be redeemed								35.00%
Minimum percentage of original aggregate principal amount of notes to remain outstanding immediately after redemption								65.00%
Interest rate on senior subordinated notes								5.00%	5.00%
Maximum number of days after the date of the closing of the equity offering that the redemption should occur								60 days
Proceeds to pay down our outstanding credit facility balance				$ 865,000,000	$ 764,826,000		$ 350,000,000
Purchase price of senior subordinate notes as a percentage of principal		101.00%		101.00%
Ratio of debt to EBITDAX												4.25	4.25	1.00	1.00
Current ratio												1.0	1.0	1.0	1.0