Long-Term Debt - Additional Information (Detail) (USD $)					9 Months Ended						3 Months Ended	9 Months Ended
	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2014 Letter of Credit [Member]	Nov. 03, 2014 Subsequent Event [Member] Letter of Credit [Member]	Sep. 30, 2014 Minimum [Member]	Sep. 30, 2014 Maximum [Member]	Sep. 30, 2014 Bank debt [Member]	Jun. 24, 2014 Bank debt [Member]	Nov. 03, 2014 Bank debt [Member] Subsequent Event [Member]	Oct. 31, 2014 Bank debt [Member] Subsequent Event [Member]	Sep. 30, 2014 1.75% Senior Convertible Notes due 2017 [Member]	Sep. 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							0	500,000,000	0	500,000,000
Outstanding borrowing under bank credit facility			480,800,000	480,800,000			19,200,000		19,200,000
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											$ 31.36	$ 31.36
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	1,037,440,000	1,027,084,000					0				262,400,000	262,400,000	252,100,000
Interest expense related to amortization of discount											3,500,000	10,400,000
Amortization of deferred financing costs											300,000	1,000,000
Interest expense related to contractual interest coupon of convertible notes											$ 1,300,000	$ 3,900,000