Long-Term Debt - Additional Information (Detail) (USD $)			1 Months Ended	3 Months Ended	6 Months Ended			6 Months Ended			6 Months Ended				3 Months Ended	6 Months Ended	3 Months Ended	6 Months Ended	3 Months Ended	6 Months Ended
	Jun. 30, 2013	Dec. 31, 2012	Jun. 28, 2013 Senior Convertible Notes due 2017	Jun. 30, 2013 Senior Convertible Notes due 2017	Jun. 30, 2013 Senior Convertible Notes due 2017	Dec. 31, 2012 Senior Convertible Notes due 2017	Mar. 06, 2012 Senior Convertible Notes due 2017	Jun. 30, 2013 Bank Debt	Dec. 31, 2012 Bank Debt	Apr. 26, 2011 Bank Debt	Jun. 30, 2013 Bank Credit Facility	Aug. 05, 2013 Bank Credit Facility Subsequent Event	Jun. 30, 2013 Letter of Credit	Aug. 05, 2013 Letter of Credit Subsequent Event	Jun. 30, 2013 8.625 % Senior Notes Due 2017	Jun. 30, 2013 8.625 % Senior Notes Due 2017	Jun. 30, 2013 Seven Point Five Percent Senior Notes Due Two Thousand Twenty Two	Jun. 30, 2013 Seven Point Five Percent Senior Notes Due Two Thousand Twenty Two	Jun. 30, 2013 7.5% Senior Notes due 2022	Jun. 30, 2013 7.5% Senior Notes due 2022	Dec. 31, 2012 7.5% Senior Notes due 2022
Debt Instrument [Line Items]
Credit facility initial date								Apr. 26, 2011
Redetermined base borrowing and credit facility										$ 700,000,000
Maturity of new credit facility if note issue under 2004 indenture are retired on or before April 15, 2014											Apr. 26, 2015
Initial bank and availability under facility								400,000,000			378,500,000	378,500,000
Outstanding borrowing under bank credit facility											0	0	21,500,000	21,500,000
Oil and gas reserve as proportion of discounted present value of future net cash flow, for mortgage								80.00%
Aggregate principal amount of Senior Subordinated Notes							300,000,000
Initial conversion rate of convertible note 2017					An initial conversion rate of 23.4449 shares of our common stock per $1,000 principal amount of 2017 Convertible Notes
Initial conversion price of convertible note 2017				$ 42.65	$ 42.65
Closing share price			$ 22.03
Payment for call option					70,800,000
Anti-dilution adjustments for purchases of call option					7,033,470
Strike price per share					$ 55.91
Proceeds from sale of warrants					40,100,000
Carrying amount of liability component				229,200,000	229,200,000
Effective market interest rate					7.51%
Carrying amount of equity component				70,800,000	70,800,000
Transaction costs allocated to liability and equity components					8,900,000
Proceeds from warrant transaction				40,100,000	40,100,000
Liability component of convertible note	920,485,000	914,126,000		245,500,000	245,500,000	239,100,000													300,000,000	300,000,000	300,000,000
Interest cost related to amortization of discount				3,200,000	6,300,000
Amortization of deferred financing cost				300,000	600,000										400,000	800,000	100,000	300,000
Interest expense related to contractual interest coupon of convertible notes				$ 1,300,000	$ 2,600,000										$ 8,100,000	$ 16,200,000			$ 5,600,000	$ 11,200,000