Long-Term Debt - Additional Information (Detail)	3 Months Ended		6 Months Ended
	Jun. 30, 2015 USD ($) $ / shares	Jun. 30, 2014 USD ($)	Jun. 30, 2015 USD ($) $ / shares shares	Jun. 30, 2014 USD ($)	Aug. 05, 2015 USD ($)	May. 01, 2015 USD ($)	Dec. 31, 2014 USD ($)	Jun. 24, 2014 USD ($)	Mar. 06, 2012 USD ($)
Debt Instrument [Line Items]
Liability component of convertible note	$ 1,048,406,000		$ 1,048,406,000				$ 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	$ 19,200,000		$ 19,200,000
Subsequent Event [Member] | Letter of Credit [Member]
Debt Instrument [Line Items]
Outstanding borrowing under bank credit facility					$ 19,200,000
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 | $ / shares	$ 1,000		$ 1,000
Initial conversion price of convertible note 2017 | $ / shares			42.65
Closing share price | $ / shares	$ 12.59		$ 12.59
Payment for call option			$ 70,800,000
Anti-dilution adjustments for purchases of call option | shares			7,033,470
Strike price per share | $ / shares			$ 55.91
Proceeds from sale of warrants			$ 40,100,000
Liability component of convertible note	$ 273,400,000		273,400,000
Interest expense related to amortization of discount	3,700,000	$ 3,500,000	7,400,000	$ 6,800,000
Amortization of deferred financing costs	400,000	300,000	700,000	600,000
Interest expense related to contractual interest coupon of convertible notes	1,300,000	$ 1,300,000	$ 2,600,000	$ 2,600,000
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	480,800,000		$ 480,800,000			$ 500,000,000
Outstanding borrowing under bank credit facility	$ 0		$ 0
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			80.00%
Bank Debt [Member] | Subsequent Event [Member]
Debt Instrument [Line Items]
Initial bank and availability under facility					480,800,000
Outstanding borrowing under bank credit facility					$ 0