Long-Term Debt - Additional Information (Detail) (USD $)	3 Months Ended		9 Months Ended			3 Months Ended	9 Months Ended	3 Months Ended	9 Months Ended		1 Months Ended	3 Months Ended	9 Months Ended			9 Months Ended			9 Months Ended		3 Months Ended	9 Months Ended	3 Months Ended	9 Months Ended	3 Months Ended	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012	Sep. 30, 2013 Letter of Credit [Member]	Sep. 30, 2013 Letter of Credit [Member]	Sep. 30, 2013 Revolving Credit Facility [Member]	Sep. 30, 2013 Revolving Credit Facility [Member]	Nov. 05, 2013 Subsequent Event [Member] Letter of Credit [Member]	Sep. 30, 2013 Senior Convertible Notes due 2017 [Member]	Sep. 30, 2013 Senior Convertible Notes due 2017 [Member]	Sep. 30, 2013 Senior Convertible Notes due 2017 [Member]	Dec. 31, 2012 Senior Convertible Notes due 2017 [Member]	Mar. 06, 2012 Senior Convertible Notes due 2017 [Member]	Sep. 30, 2013 Bank Debt [Member]	Dec. 31, 2012 Bank Debt [Member]	Apr. 26, 2011 Bank Debt [Member]	Sep. 30, 2013 Bank Credit Facility [Member]	Nov. 05, 2013 Bank Credit Facility [Member] Subsequent Event [Member]	Sep. 30, 2013 8.625 % Senior Notes Due 2017 [Member]	Sep. 30, 2013 8.625 % Senior Notes Due 2017 [Member]	Sep. 30, 2013 7.5% Senior Notes Due 2022 [Member]	Sep. 30, 2013 7.5% Senior Notes Due 2022 [Member]	Sep. 30, 2013 7.5% Senior Notes due 2022 [Member]	Sep. 30, 2013 7.5% Senior Notes due 2022 [Member]	Dec. 31, 2012 7.5% Senior Notes due 2022 [Member]
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						21,500,000	21,500,000			21,500,000									0	0
Interest expense related to fees on bank credit facility and letters of credit	7,922,000	7,692,000	26,452,000	21,107,000		100,000	400,000	500,000	1,400,000
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 rate of common stock													23.4449
Principal amount of 2017 Convertible Notes											$ 1,000	$ 1,000	$ 1,000
Initial conversion price of convertible note 2017											$ 42.65	$ 42.65	$ 42.65
Closing share price											$ 32.43
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
Liability component of convertible note	923,745,000		923,745,000		914,126,000						248,700,000	248,700,000	248,700,000	239,100,000											300,000,000	300,000,000	300,000,000
Interest cost related to amortization of discount												3,300,000	9,600,000
Amortization of deferred financing cost												300,000	900,000								400,000	1,200,000	200,000	500,000
Interest expense related to contractual interest coupon of convertible notes												$ 1,300,000	$ 3,900,000								$ 8,100,000	$ 24,300,000			$ 5,600,000	$ 16,800,000