Long - Term Debt - Additional Information (Detail) (USD $)	9 Months Ended				9 Months Ended	1 Months Ended		9 Months Ended			9 Months Ended
	Sep. 30, 2012	Sep. 30, 2012 Aggregate [Member]	Sep. 30, 2012 Maximum [Member]	Sep. 30, 2012 Minimum [Member]	Sep. 30, 2012 Bank debt	Sep. 30, 2012 Senior Convertible Notes due 2017	Sep. 28, 2012 Senior Convertible Notes due 2017	Sep. 30, 2012 Senior Convertible Notes due 2017	Jun. 30, 2012 Senior Convertible Notes due 2017	Mar. 06, 2012 Senior Convertible Notes due 2017	Sep. 30, 2012 Bank Credit Facility	Nov. 02, 2012 Bank Credit Facility Subsequent Event [Member]	Sep. 30, 2012 Letter of Credit	Oct. 22, 2012 Senior Notes due 2022 [Member] Subsequent Event [Member]	Oct. 22, 2012 Senior Notes due 2014 [Member] Subsequent Event [Member]
Debt Instrument [Line Items]
Credit facility initial date					Apr. 26, 2011
Redetermined base borrowing and Credit Facility					$ 700,000,000
Bank credit facility maturing with Syndicated Bank Group					Sep. 15, 2014
Maturity of new credit facility if note issue under 2004 indenture are retired on or before April 15, 2014					Apr. 15, 2014						Apr. 26, 2015
Initial bank and availability under facility					400,000,000						379,000,000
Outstanding borrowing under bank credit facility											0	0	21,000,000
Value of public offering														300,000,000
Repayment of aggregate principal amount															200,000,000
Borrowing base rate	30.00%
Increase in outstanding notes		900,000,000	1,250,000,000	1,100,000,000
Oil and gas reserve as a proportion of discounted present value of the future net cash flow, for Mortgage					80.00%
Aggregate principal amount of Senior Subordinated Notes										300,000,000
Proceeds from issuance of senior notes								291,100,000
Interest rate due on convertible note due 2017										1.75%
Initial conversion rate of convertible note 2017								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							$ 25.12
Convertible notes principal amount						1,000		1,000
Percentage of current conversion price lower than closing sale price									130.00%
Maximum trading day									20 days
Consecutive trading days ending on last trading day									30 days
Period in which distribution is made to all holders of common stock rights, option or warrants								45 days
Consecutive trading days immediately preceding, but excluding, declaration date								10 days
Percentage in which distribution has per share value exceeding closing sale price of common stock								10.00%
Percentage in which closing sale price of common stock is excess of convertible notes								98.00%
Number of consecutive business day								5 days
Number of consecutive trading day								25
Percentage of convertible notes principal amount								100.00%
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 sold warrants								40,100,000
Carrying amount of liability component						229,200,000		229,200,000
Carrying amount of equity component						70,800,000		70,800,000
Transaction costs allocated to liability and equity components								$ 8,900,000
Effective market interest rate						7.51%