Long-Term Debt - Additional Information (Detail) (USD $)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014	May 04, 2015	Jun. 24, 2014	May 01, 2015	Mar. 06, 2012
Debt Instrument [Line Items]
Liability component of convertible note	1,044,675,000		$ 1,041,035,000
Minimum [Member]
Debt Instrument [Line Items]
Percentage of borrowing base utilization	1.50%
Maximum [Member]
Debt Instrument [Line Items]
Percentage of borrowing base utilization	2.50%
Letter of Credit [Member]
Debt Instrument [Line Items]
Outstanding borrowing under bank credit facility		480,800,000
Subsequent Event [Member] | Letter of Credit [Member]
Debt Instrument [Line Items]
Outstanding borrowing under bank credit facility				480,800,000
Bank debt [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
Outstanding borrowing under bank credit facility		19,200,000
Period in which outstanding amount has to be repaid to cure deficiency	10 days
Period in which bank has to add new properties to borrowing base and has to grant mortgage to banks	30 days
Oil and gas reserve as proportion of discounted present value of future net cash flow, for mortgage	80
Bank debt [Member] | Subsequent Event [Member]
Debt Instrument [Line Items]
Initial bank and availability under facility				0		500,000,000
Outstanding borrowing under bank credit facility				480,800,000
1.75% Senior Convertible Notes due 2017 [Member]
Debt Instrument [Line Items]
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
Initial conversion price of convertible note 2017	42.65
Closing share price	14.68
Payment for call option	70,800,000
Anti-dilution adjustments for purchases of call option	7,033,470	7,033,470
Strike price per share		$ 55.91
Proceeds from sale of warrants		40,100,000
Liability component of convertible note	269,700,000		266,000,000
Interest expense related to amortization of discount	3,600,000	3,400,000
Amortization of deferred financing costs	300,000	300,000
Interest expense related to contractual interest coupon of convertible notes	1,300,000	$ 1,300,000