Indebtedness - Additional Information (Detail) (USD $)	3 Months Ended		6 Months Ended				1 Months Ended		6 Months Ended				6 Months Ended
	Jun. 30, 2013 CommercialBank	Jun. 30, 2012	Jun. 30, 2013 CommercialBank	Jun. 30, 2012	Apr. 08, 2013	Dec. 31, 2012	Apr. 30, 2013 7.25% Senior Subordinated notes due 2018	Jun. 30, 2013 5.00% Senior Subordinated notes due 2023	Jun. 30, 2013 5.00% Senior Subordinated notes due 2023	Dec. 31, 2012 5.00% Senior Subordinated notes due 2023	Jun. 30, 2013 5.00% Senior Subordinated notes due 2023 Senior note redeemed in March 15, 2018	Jun. 30, 2013 5.00% Senior Subordinated notes due 2023 Senior note redeemed in March 15, 2021	Jun. 30, 2013 5.00% Senior Subordinated notes due 2023 Senior note redeemed in March 15, 2016	Jun. 30, 2013 Minimum Ratio	Jun. 30, 2013 Maximum Ratio
Debt Instrument [Line Items]
Interest capitalized during the period	$ 0	$ 0	$ 0	$ 0
Bank Credit facility, Facility amount	1,750,000,000		1,750,000,000		1,750,000,000
Bank Credit facility, Borrowing base	2,000,000,000		2,000,000,000		2,000,000,000
Percentage holding of commercial banks, Maximum	9.00%		9.00%
Number of commercial banks included in current bank group	28		28
Notice period for increase in facility amount	20 days
Bank debt	309,000,000		309,000,000			739,000,000
Undrawn letters of credit	84,700,000		84,700,000
Borrowing capacity available under the facility amount	1,400,000,000		1,400,000,000
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	2.20%	2.70%	2.10%	2.30%
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%		1.50%
Interest rate margin on base rate loans	0.50%		0.50%
Aggregate principal amount of senior subordinated notes								750,000,000	750,000,000
Interest rate on senior subordinated notes								5.00%	5.00%	5.00%
Net proceeds from issuance of subordinated notes									738,800,000
Deferred Finance Costs								11,200,000	11,200,000
Aggregate principal amount of senior subordinated notes exchanged								0
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%		101.00%
Announced call for redemption amount of debt							250,000,000
Loss on early extinguishment of debt	$ 12,280,000		$ 12,280,000
Debt redemption price percent of par							103.625%
Ratio of debt to EBITDAX														1.0	4.25
Current ratio														1.0	1.0