Summary of Significant Accounting Policies - Additional Information (Detail)	3 Months Ended		9 Months Ended
	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) Segment	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Accounts receivable	$ 77,154,000		$ 77,154,000		$ 119,332,000
Depreciation, depletion, amortization and accretion			$ 114,331,000	$ 98,672,000
Number of operating segment | Segment			1
Oil and Gas Properties [Member]
Summary Of Significant Accounting Policies [Line Items]
Depreciation, depletion, amortization and accretion	44,900,000	$ 34,000,000.0	$ 112,400,000	97,300,000
Other property and equipment [Member]
Summary Of Significant Accounting Policies [Line Items]
Depreciation	600,000	400,000	$ 1,600,000	1,400,000
Other property and equipment [Member] | Minimum [Member]
Summary Of Significant Accounting Policies [Line Items]
Property and equipment, expected lives			5 years
Other property and equipment [Member] | Maximum [Member]
Summary Of Significant Accounting Policies [Line Items]
Property and equipment, expected lives			40 years
Proved Oil And Gas Properties [Member] | Marcellus Shale [Member]
Summary Of Significant Accounting Policies [Line Items]
Impairment of oil and gas properties	0	0	$ 0	0
Unproved Oil And Gas Properties [Member]
Summary Of Significant Accounting Policies [Line Items]
Impairment of oil and gas properties	14,100,000	$ 7,000,000.0	36,200,000	$ 20,600,000
Commodity Contract [Member] | Not Designated as Hedging Instrument [Member]
Summary Of Significant Accounting Policies [Line Items]
Fair value of commodity derivative contracts	28,100,000		28,100,000		5,700,000
Revenue From Contract With Customer [Member]
Summary Of Significant Accounting Policies [Line Items]
Accounts receivable	$ 57,200,000		$ 57,200,000		$ 94,100,000