Long-Term Debt - Additional Information (Detail) (USD $)	9 Months Ended	3 Months Ended		9 Months Ended	12 Months Ended		9 Months Ended					9 Months Ended					3 Months Ended
	Sep. 30, 2013	Sep. 30, 2013 Senior Notes due 2020 [Member]	Sep. 30, 2013 Revolving credit facility	Sep. 30, 2013 Revolving credit facility	Dec. 31, 2012 Revolving credit facility	Oct. 28, 2013 Revolving credit facility	Sep. 30, 2013 Revolving credit facility Minimum	Sep. 30, 2013 Revolving credit facility Maximum	Sep. 30, 2013 Revolving credit facility through December 31, 2013 Maximum	Sep. 30, 2013 Revolving credit facility through June 30, 2014 Maximum	Sep. 30, 2013 Revolving credit facility through maturity Maximum	Sep. 30, 2013 Revolving credit facility Group Twelve	Sep. 30, 2013 Revolving credit facility Group Thirteen	Sep. 30, 2013 Revolving credit facility Letter of Credit	Oct. 28, 2013 Previous Revolving Credit Facility	Sep. 30, 2013 Previous Revolving Credit Facility	Sep. 30, 2013 10.375% Senior Unsecured Notes due 2016	Dec. 31, 2012 10.375% Senior Unsecured Notes due 2016	Sep. 30, 2013 7.25% Senior Unsecured Notes due 2019	Dec. 31, 2012 7.25% Senior Unsecured Notes due 2019	Feb. 29, 2012 7.25% Senior Unsecured Notes due 2019	Sep. 30, 2013 7.25% Senior Unsecured Notes due 2019 Group One	Sep. 30, 2013 7.25% Senior Unsecured Notes due 2019 Group Two	Sep. 30, 2013 Senior Notes due 2020 [Member]	Dec. 31, 2012 Senior Notes due 2020 [Member]
Debt Disclosure [Line Items]
Senior Notes, Noncurrent																	$ 0	$ 294,759,000	$ 300,000,000	$ 300,000,000				$ 775,000,000	$ 0
Line Of Credit Facility Borrowing Base			350,000,000	350,000,000											425,000,000
Covenant requirement, leverage ratio			4,500
Maximum borrowing capacity						400,000,000								20,000,000
Optional additional borrowing			200,000,000	200,000,000
Letter of credit amount outstanding																3,000,000
Available borrowing capacity																219,000,000
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			1.75%	1.75%			0.50%	1.50%
Commitment fees for undrawn credit facility												0.375%	0.50%
LIne of Credit Facility, Commitment Fee Percentage at Period End			0.375%	0.375%
Line of Credit Facility, Interest Rate at Period End			1.9375%	1.9375%
Line of Credit Facility, Interest Rate at Period End, LIBOR component			0.1875%	0.1875%
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%			0.00%
Redemption option, percent of principal																						103.625%	100.00%
Debt Instrument, Face Amount																	0	300,000,000
Debt Proceeds Used For Cash Consideration, Business Acquisition		$ 380,000,000