Long-Term Debt - Additional Information (Detail) (USD $)	3 Months Ended		9 Months Ended			24 Months Ended	3 Months Ended	9 Months Ended		9 Months Ended		0 Months Ended		24 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014 Senior Unsecured Debt [Member]	Dec. 31, 2012 Senior Unsecured Debt [Member]	Sep. 30, 2014 Second Amended Credit Facility [Member]	Sep. 30, 2014 Second Amended Credit Facility [Member]	Mar. 27, 2014 Second Amended Credit Facility [Member]	Sep. 30, 2014 Current Ratio [Member] Second Amended Credit Facility [Member]	Sep. 30, 2014 Minimum [Member] Second Amended Credit Facility [Member]	Sep. 24, 2014 2022 Notes [Member] Senior Unsecured Debt [Member]	Sep. 24, 2013 2022 Notes [Member] Senior Unsecured Debt [Member]	Dec. 31, 2012 2019 Notes [Member] Senior Unsecured Debt [Member]	Dec. 31, 2012 2021 Notes [Member] Senior Unsecured Debt [Member]	Dec. 31, 2012 2023 Notes [Member] Senior Unsecured Debt [Member]	Dec. 31, 2013 2023 Notes [Member] Senior Unsecured Debt [Member]
Debt Disclosure [Line Items]
Principal debt amount													$ 1,000,000,000.0	$ 400,000,000.0	$ 400,000,000.0		$ 400,000,000.0
Stated interest rate													6.875%	7.25%	6.50%		6.875%
Debt instrument, maturity date								Apr. 05, 2018				Mar. 15, 2022		Feb. 01, 2019	Nov. 01, 2021	Jan. 15, 2023
Net proceeds from issuance of notes						1,175,800,000						983,600,000
Line of credit facility maximum increase to aggregate commitments							2,000,000,000.0	2,000,000,000.0	1,750,000,000.0
Senior unsecured notes payment frequency					semi-annually
Redemption percentage, maximum					35.00%
Redemption term					180 days
Percentage of minimum outstanding principal amount					65.00%
Event of default by payment default ($10.0 million or more)					10,000,000.0
Event of default by final judgments (in excess of $10.0 million)					10,000,000.0
Line of credit facility, maximum borrowing capacity							1,500,000,000.0	1,500,000,000.0
Line of credit facility, capacity available for specific purpose other than for trade purchases							2,500,000,000	2,500,000,000
Percentage of the reserve value of assets as determined by reserve reports (at least 80%)											80.00%
Commitment fee of borrowing base capacity not utilized							0.375%
Ratio of EBITDAX to interest expense necessary to maintain covenants											2.5
Minimum current ratio										1.0
Line of credit facility, amount outstanding							350,000,000	350,000,000
Outstanding letters of credit							5,200,000	5,200,000
Line of credit facility, remaining borrowing capacity							1,144,800,000	1,144,800,000
Weighted average interest rate							1.70%	1.70%
Deferred financing costs	37,100,000		37,100,000
Amortization of deferred financing costs	$ 1,600,000	$ 1,000,000	$ 4,800,000	$ 2,900,000