Long-Term Debt - Additional Information (Detail) (USD $)	12 Months Ended			12 Months Ended					12 Months Ended						12 Months Ended
	Dec. 31, 2012	Dec. 31, 2012 Revolving credit facility	Mar. 31, 2013 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	Mar. 31, 2013 Revolving credit facility Letter of Credit	Mar. 31, 2013 Previous Revolving Credit Facility	Dec. 31, 2012 Previous Revolving Credit Facility	Dec. 31, 2012 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 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	May 01, 2013 Subsequent Event Revolving credit facility
Debt Disclosure [Line Items]
Maximum borrowing capacity			$ 300,000,000								$ 20,000,000										$ 276,200,000
Optional additional borrowing			300,000,000
Initial borrowing capacity												300,000,000
Decrease in borrowing capacity																					23,800,000
Letter of credit amount outstanding													2,800,000
Available borrowing capacity													$ 259,200,000
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%
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
Original sale amount, percentage of par														97.00%
Effective yield to maturity														11.00%
Annual interest rate														10.375%			7.25%	7.25%
Interest payment date															June 15	December 15
Redemption option, percent of principal																			103.625%	100.00%