Acquisitions and Divestitures - Additional Information (Detail) (USD $)	3 Months Ended	6 Months Ended		12 Months Ended		3 Months Ended	6 Months Ended	3 Months Ended			3 Months Ended
	Jun. 30, 2014	Jun. 30, 2014	Jun. 30, 2013	Dec. 31, 2013	Jan. 31, 2014	Jun. 30, 2014 Selma Chalk	Jun. 30, 2014 East Texas Gathering Assets [Member]	Sep. 30, 2014 Subsequent Event [Member] acre	Sep. 30, 2014 Subsequent Event [Member] Eagle Ford Shale, Lavaca County [Member]	Jul. 31, 2014 Subsequent Event [Member] Eagle Ford Shale, Lavaca County [Member]	Sep. 30, 2014 Subsequent Event [Member] South Texas Oil Gathering Sys. Development Rights	Sep. 30, 2014 Subsequent Event [Member] Selma Chalk
Business Acquisition [Line Items]
Payments To Settle Obligations Assumed In Acquisition Net		$ 0	$ 36,310,000	$ (22,500,000)
Purchased Oil and Gas Interests, Gross Area Of Property Acquired								13,125
Proceeds from Sale of Intangible Assets											147,000,000
Impairment of Oil and Gas Properties						117,900,000
Business Combination, Disputed Contractual Adjustments, Settled in Arbitration								31,000,000
Business Combination, Revenue Suspense Funds, Released in Arbitration, Subsequent Years								2,700,000
Proceeds from sale of legacy natural gas assets in West Virginia, Kentucky and Virginia						72,700,000	96,000,000					71,000,000
Gain (Loss) on Sale of Oil and Gas Property	67,300,000
Sale Leaseback Transaction, Recognized Gain, Net	56,700,000
Sale Leaseback Transaction, Deferred Gain, Gross					10,600,000
Sale Leaseback Transaction, Current Period Gain Recognized	100,000	200,000
Sale Leaseback Transaction, Deferred Gain, Current	400,000	400,000
Sale Leaseback Transaction, Deferred Gain, Net	10,000,000	10,000,000
Purchased Oil and Gas Interests, Net Area Of Property Acquired								11,660
Asset Acquisition, Purchase Price										45,000,000
Business Combination, Consideration Transferred									34,000,000
Business Combination, Consideration Transferred, Liabilities Incurred									$ 11,000,000