Long-Term Debt - Additional Information (Detail) (USD $)				6 Months Ended			6 Months Ended			3 Months Ended	6 Months Ended
	Jun. 30, 2014	Dec. 31, 2013	Jun. 30, 2014 Letter of Credit [Member]	Jun. 30, 2014 Minimum [Member]	Jun. 30, 2014 Maximum [Member]	Aug. 05, 2014 Subsequent Event [Member] Letter of Credit [Member]	Jun. 30, 2014 Bank debt [Member]	Jun. 24, 2014 Bank debt [Member]	Aug. 05, 2014 Bank debt [Member] Subsequent Event [Member]	Jun. 30, 2014 1.75% Senior Convertible Notes due 2017 [Member]	Jun. 30, 2014 1.75% Senior Convertible Notes due 2017 [Member]	Dec. 31, 2013 1.75% Senior Convertible Notes due 2017 [Member]	Mar. 06, 2012 1.75% Senior Convertible Notes due 2017 [Member]
Debt Instrument [Line Items]
Credit facility initial date							Jun. 24, 2014
Redetermined base borrowing and credit facility								$ 900,000,000
Maturity of new credit facility if note issue under 2004 indenture are retired on or before April 15, 2014							Jul. 01, 2019
Initial bank and availability under facility							479,000,000	500,000,000	479,000,000
Additional borrowing from the previous base								400,000,000
Outstanding borrowing under bank credit facility			21,000,000			21,000,000	0		0
Oil and gas reserve as proportion of discounted present value of future net cash flow, for mortgage							80
Percentage of borrowing base utilization				1.50%	2.50%
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											0.0234449
Principal amount of 2017 convertible notes										$ 1,000	$ 1,000
Initial conversion price of convertible note 2017											$ 42.65
Closing share price											$ 46.79
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	1,033,931,000	1,027,084,000					0			258,900,000	258,900,000	252,100,000
Interest expense related to amortization of discount										3,500,000	6,800,000
Amortization of deferred financing costs										300,000	600,000
Interest expense related to contractual interest coupon of convertible notes										$ 1,300,000	$ 2,600,000