Long-Term Debt - Additional Information (Detail) (USD $)	6 Months Ended	3 Months Ended		6 Months Ended	12 Months Ended	6 Months Ended					6 Months Ended				3 Months Ended
	Jun. 30, 2013	Jun. 30, 2013 Senior Notes due 2020 [Member]	Apr. 24, 2013 Senior Notes due 2020 [Member]	Jun. 30, 2013 Revolving credit facility	Dec. 31, 2012 Revolving credit facility	Jun. 30, 2013 Revolving credit facility Minimum	Jun. 30, 2013 Revolving credit facility Maximum	Jun. 30, 2013 Revolving credit facility through December 31, 2013 Maximum	Jun. 30, 2013 Revolving credit facility through June 30, 2014 Maximum	Jun. 30, 2013 Revolving credit facility through maturity Maximum	Jun. 30, 2013 Revolving credit facility Group Twelve	Jun. 30, 2013 Revolving credit facility Group Thirteen	Jun. 30, 2013 Revolving credit facility Letter of Credit	Jun. 30, 2013 Previous Revolving Credit Facility	Jun. 30, 2013 10.375% Senior Unsecured Notes due 2016	Dec. 31, 2012 10.375% Senior Unsecured Notes due 2016	Jun. 30, 2013 7.25% Senior Unsecured Notes due 2019	Feb. 29, 2012 7.25% Senior Unsecured Notes due 2019	Jun. 30, 2013 7.25% Senior Unsecured Notes due 2019 Group One	Jun. 30, 2013 7.25% Senior Unsecured Notes due 2019 Group Two	Jun. 30, 2013 Senior Notes due 2020 [Member]
Debt Disclosure [Line Items]
Maximum borrowing capacity				$ 350,000,000									$ 20,000,000
Line of Credit Facility, Current Borrowing Capacity														276,200,000
Optional additional borrowing				250,000,000
Initial borrowing capacity														350,000,000
Letter of credit amount outstanding														3,000,000
Available borrowing capacity														280,000,000
Credit facility interest rate description					Borrowings under the Revolver bear interest, at our option, at either (i) a rate derived from the London Interbank Offered Rate, as adjusted for statutory reserve requirements for Eurocurrency liabilities (the “Adjusted LIBOR”), plus an applicable margin ranging from 1.500% to 2.500% or (ii) the greater of (a) the prime rate, (b) the federal funds effective rate plus 0.5% or (c) the one-month Adjusted LIBOR plus 1.0%, and, in each case, plus an applicable margin (ranging from 0.500% to 1.500%). The applicable margin is determined based on the ratio of our outstanding borrowings to the available Revolver capacity.
Credit facility interest rate option one, applicable margin rate over Adjusted LIBOR						1.50%	2.50%
Credit facility interest rate option two, base rate over federal funds effective rate				0.50%
Debt Instrument, Description of Variable Rate Basis	one-month Adjusted LIBOR
Credit facility interest rate option two, base rate over one-month Adjusted LIBOR				1.00%
Credit facility interest rate option two, applicable margin rate						0.50%	1.50%
Commitment fees for undrawn credit facility											0.375%	0.50%
Line of Credit Facility, Interest Rate at Period End				1.75%
Current ratio						1.0
Debt to EBITDAX ratio								4.5	4.25	4.0
Debt Tender Offer Payments															330,900,000
Gain Loss On Extinguishment Of Debt Gross															29,200,000
Write off of Deferred Debt Issuance Cost															10,000,000
Annual interest rate																	7.25%	7.25%			8.50%
Redemption option, percent of principal																			103.625%	100.00%
Debt Instrument, Face Amount			775,000,000												0	300,000,000
Debt Proceeds Used For Cash Consideration, Business Acquisition		$ 380,000,000