Long-Term Debt - Additional Information (Detail) (USD $)	1 Months Ended	12 Months Ended					12 Months Ended					12 Months Ended							12 Months Ended								1 Months Ended	12 Months Ended
	Sep. 30, 2012	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012 Revolving credit facility	Sep. 30, 2012 Revolving credit facility	Dec. 31, 2012 Revolving credit facility Minimum	Dec. 31, 2012 Revolving credit facility Maximum	Dec. 31, 2012 Revolving credit facility through December 31, 2013 Maximum	Dec. 31, 2012 Revolving credit facility through June 30, 2014 Maximum	Dec. 31, 2012 Revolving credit facility through maturity Maximum	Dec. 31, 2012 Revolving credit facility Group Twelve	Dec. 31, 2012 Revolving credit facility Group Thirteen	Sep. 30, 2012 Revolving credit facility Letter of Credit	Dec. 31, 2012 Previous Revolving Credit Facility	Sep. 30, 2012 Previous Revolving Credit Facility	Dec. 31, 2012 10.375% Senior Unsecured Notes due 2016	Dec. 31, 2009 10.375% Senior Unsecured Notes due 2016	Dec. 31, 2012 10.375% Senior Unsecured Notes due 2016 Semi Annual Payment, First Payment	Dec. 31, 2012 10.375% Senior Unsecured Notes due 2016 Semi Annual Payment, Second Payment	Dec. 31, 2012 10.375% Senior Unsecured Notes due 2016 Group One	Dec. 31, 2012 10.375% Senior Unsecured Notes due 2016 Group Two	Dec. 31, 2012 7.25% Senior Unsecured Notes due 2019	Feb. 29, 2012 7.25% Senior Unsecured Notes due 2019	Dec. 31, 2012 7.25% Senior Unsecured Notes due 2019 Group One	Dec. 31, 2012 7.25% Senior Unsecured Notes due 2019 Group Two	Apr. 30, 2011 4.50% Convertible Notes due 2012	Dec. 31, 2011 4.50% Convertible Notes due 2012
Debt Disclosure [Line Items]
Maximum borrowing capacity						$ 300,000,000								$ 20,000,000
Optional additional borrowing						300,000,000
Debt Instrument Initial Borrowing Capacity Amount																300,000,000
Letter of credit amount outstanding															2,100,000
Available borrowing capacity															297,900,000
Debt issuance costs paid	2,000,000	2,032,000	8,854,000	0
Debt instrument, debt issuance costs, amortization period	5 years
Write off of deferred debt issuance cost	3,200,000
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%
Current ratio							1
Debt to EBITDAX ratio									4.5	4.25	4
Original sale amount, percentage of par																	97.00%
Effective yield to maturity																	11.00%
Annual interest rate																	10.375%	10.375%					7.25%	7.25%			4.50%
Interest payment date																			June 15	December 15
Redemption period start																					June 2013	June 2015
Redemption option, percent of principal																					105.188%	100.00%			103.625%	100.00%
Principal amount of Convertible notes tendered																											225,100,000
Cash paid to repurchase principal																											233,000,000
Tender premium per principal amount																											35
Tender offer, principal amount																											1,000
Percentage of outstanding debt extinguished																											98.00%
Loss On extinguishment of debt ross																												25,900,000
Loss on extinguishment of debt		3,164,000	25,421,000	0																								24,200,000
Loss On extinguishment of debt recognized in shareholders equity																												$ 1,700,000