Indebtedness - Additional Information (Detail) (USD $)	3 Months Ended			12 Months Ended									3 Months Ended	12 Months Ended					12 Months Ended
	Dec. 31, 2012 CommercialBank	Sep. 30, 2011	Jun. 30, 2011	Dec. 31, 2012 CommercialBank	Dec. 31, 2011	Dec. 31, 2010	Jun. 24, 2011 6.375% Senior Subordinated notes due 2015 [Member]	May 31, 2011 6.375% Senior Subordinated notes due 2015 [Member]	May 25, 2011 6.375% Senior Subordinated notes due 2015 [Member]	Jun. 24, 2011 7.5% senior subordinated notes due 2016, net of discount [Member]	May 31, 2011 7.5% senior subordinated notes due 2016, net of discount [Member]	May 25, 2011 7.5% senior subordinated notes due 2016, net of discount [Member]	Mar. 31, 2012 5.00% Senior Subordinated notes due 2022 [Member]	Dec. 31, 2012 5.00% Senior Subordinated notes due 2022 [Member]	Dec. 31, 2011 5.00% Senior Subordinated notes due 2022 [Member]	Dec. 31, 2012 5.00% Senior Subordinated notes due 2022 [Member] Subsequent Event [Member]	Mar. 31, 2012 5.00% Senior Subordinated notes due 2022 [Member] Subsequent Event [Member]	Dec. 31, 2012 7.5% senior subordinated notes due 2017 [Member]	Dec. 31, 2012 Minimum [Member] Ratio	Dec. 31, 2012 Maximum [Member] Ratio
Debt Instrument [Line Items]
Interest costs, capitalized during the period				$ 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
Outstanding balance under the bank credit facility	739,000,000			739,000,000	187,000,000
Undrawn letters of credit	84,700,000			84,700,000
Borrowing capacity available under the facility amount	926,300,000			926,300,000
Maturity date of loan				Feb. 18, 2016
Spread for Alternate Base Rate																			0.50%	1.50%
Spread for LIBOR Rate																			1.50%	2.50%
Weighted average interest rate on the bank credit facility				2.20%	2.20%	2.20%
Annual rate of commitment fee paid on the undrawn balance																			0.375%	0.50%
Commitment fee				0.375%
Interest rate margin on LIBOR loans	1.75%			1.75%
Interest rate margin on base rate loans	0.75%			0.75%
Aggregate principal amount of senior subordinated notes								150,000,000			250,000,000		600,000,000
Net proceeds after underwriting discounts and commissions				600,000,000	500,000,000	500,000,000							589,500,000
Discount on Senior Subordinated notes													10,500,000
Maximum redemption price of notes as percentage of principal amount													102.50%	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
Purchase price of senior subordinate notes as a percentage of principal	101.00%			101.00%			102.125%		102.375%	103.75%		104.00%						103.75%
Gains (Losses) on early extinguishment of debt	(11,063,000)	4,000	(18,580,000)	(11,063,000)	(18,576,000)	(5,351,000)
Purchase price of remaining outstanding senior subordinate notes							$ 41,100,000		$ 108,900,000	$ 51,200,000		$ 198,800,000
Ratio of debt to EBITDAX																			1.00	4.25
Current ratio																			1.0	1.0