Indebtedness - Additional Information (Detail) (USD $)	3 Months Ended				3 Months Ended		1 Months Ended					3 Months Ended
	Mar. 31, 2013 CommercialBank	Mar. 31, 2012	Apr. 08, 2013	Dec. 31, 2012	Mar. 31, 2013 5.00% Senior Subordinated notes due 2023	Dec. 31, 2012 5.00% Senior Subordinated notes due 2023	Apr. 30, 2013 7.25% Senior Subordinated notes due 2018	Mar. 31, 2013 7.25% Senior Subordinated notes due 2018	Dec. 31, 2012 7.25% Senior Subordinated notes due 2018	Mar. 31, 2013 Senior note redeemed in March 15, 2018 5.00% Senior Subordinated notes due 2023	Mar. 31, 2013 Senior note redeemed in March 15, 2021 5.00% Senior Subordinated notes due 2023	Mar. 31, 2013 Senior note redeemed in March 15, 2016 5.00% Senior Subordinated notes due 2023	Mar. 31, 2013 Minimum Ratio	Mar. 31, 2013 Maximum Ratio
Debt Instrument [Line Items]
Interest capitalized during the period	$ 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
Percentage holding of commercial banks, Maximum	9.00%
Number of commercial banks included in current bank group	28
Notice period for increase in facility amount	20 days
Bank debt	47,000,000			739,000,000
Undrawn letters of credit	84,700,000
Borrowing capacity available under the facility amount	1,600,000,000
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	2.10%	2.10%
Annual rate of commitment fee paid on the undrawn balance													0.375%	0.50%
Commitment fee	0.375%
Interest rate margin on LIBOR loans	1.50%
Interest rate margin on base rate loans	0.50%
Aggregate principal amount of senior subordinated notes					750,000,000
Interest rate on senior subordinated notes					5.00%	5.00%		7.25%	7.25%
Issuance of subordinated notes	750,000,000	600,000,000			738,800,000
Discount on Senior Subordinated notes					11,200,000
Maximum redemption price of notes as percentage of principal amount										102.50%	100.00%	105.00%
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%
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%
Announced call for redemption amount of debt							$ 250,000,000
Debt redemption price percent of par							103.625%
Ratio of debt to EBITDAX													1.0	4.25
Current ratio													1.0	1.0