Acquisitions and Divestitures - Additional Information (Detail) (USD $) Share data in Millions, except Per Share data, unless otherwise specified	0 Months Ended	3 Months Ended		4 Months Ended	12 Months Ended			1 Months Ended		1 Months Ended	3 Months Ended	12 Months Ended	1 Months Ended	3 Months Ended	12 Months Ended	3 Months Ended
	Apr. 24, 2013	Dec. 31, 2013	Jun. 30, 2013	Apr. 24, 2013	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2011 Counties of Butler and Armstrong in Pennsylvania	Dec. 31, 2012 Counties of Butler and Armstrong in Pennsylvania acre	Jul. 31, 2012 Appalachian Region except Marcellus Shale	Jun. 30, 2012 Appalachian Region except Marcellus Shale	Dec. 31, 2013 Appalachian Region except Marcellus Shale	Aug. 31, 2011 Arkoma Divestiture Restructuring	Jun. 30, 2011 Arkoma Divestiture Restructuring	Dec. 31, 2012 Arkoma Divestiture Restructuring	Dec. 31, 2013 Subsequent Event South Texas Gathering Assets	Dec. 31, 2013 Senior Notes Due 2020 [Member]	Dec. 31, 2012 Senior Notes Due 2020 [Member]
Business Acquisition [Line Items]
Payments to Acquire Businesses, Gross	$ 380,000,000
Proved	277,888,000			277,888,000
Unproved	119,709,000			119,709,000
Business Acquisition, Equity Interest Issued or Issuable, Number of Shares					10
Share Price	$ 4.23			$ 4.23
Preferential Purchase Right Settlement			21,000,000
Business Combination, Acquired Receivables, Fair Value	97,145,000			97,145,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Other	2,068,000			2,068,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets	496,810,000			496,810,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Accounts Payable	94,771,000			94,771,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Other	1,500,000			1,500,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities	96,271,000			96,271,000
Business Combination, Acquisition Related Costs					2,600,000
Business Combination, Consideration Transferred	400,539,000			400,539,000
Payments to Acquire Businesses, Net of Cash Acquired	358,239,000				(358,239,000)	0
Business Combination, Consideration Transferred, Equity Interests Issued and Issuable	42,300,000				42,300,000	0	0
Proceeds from sale of oil and gas property										100,000,000						94,000,000
Proceeds from sale net transaction costs and closing adjustments								8,100,000				95,700,000
Gain on sale of oil and gas property								3,300,000				3,300,000
Impairment of property											28,600,000			71,000,000	71,100,000
Number of net acres sold									2,700
Value of property interests being sold													30,000,000
Business Combination, Consideration Transferred	400,539,000
Business Combination Purchase Price Adjustments				19,000,000
Business Combination, Undisputed Contractual Adjustments, in Arbitration					26,500,000
Net Payments on Post Closing Adjustments from Property Sales in Prior Years					0
Loss (gain) on sales of assets, net					266,000	(4,282,000)	(2,474,000)
Gain (Loss) on Disposition of Property, Plant and Equipment due to Post-close Adjustment						1,000,000	(700,000)
Net Proceeds from Post Closing Adjustments from Property Sales in Prior Years						1,000,000	1,200,000
Senior Notes, Noncurrent																	775,000,000	0
Annual interest rate																	8.50%
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Property, Plant, and Equipment		14,000,000
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Financial Assets		46,000,000
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Financial Liabilities		$ 32,000,000