Summary of Significant Accounting Policies - Additional Information 1 (Detail) - USD ($)		3 Months Ended								12 Months Ended
		Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Summary Of Significant Accounting Policies [Line Items]
Net book value of unproved properties	[1]	$ 2,110,277,000				$ 2,644,000,000				$ 2,110,277,000	$ 2,644,000,000	$ 2,923,803,000
Abandonment and impairment of unproved properties		441,750,000	$ 6,549,000	$ 54,922,000	$ 11,773,000	217,544,000	$ 42,568,000	$ 5,193,000	$ 4,420,000	514,994,000	269,725,000	30,076,000
Depreciation expense										6,000,000	7,700,000	8,400,000
Marketable securities held in deferred compensation plans		57,300,000				67,100,000				57,300,000	67,100,000
Other investments including surface acreage		$ 9,100,000				$ 9,600,000				9,100,000	9,600,000
MRD Merger
Summary Of Significant Accounting Policies [Line Items]
Abandonment and impairment of unproved properties										436,000,000
Natural Gas and Oil Properties | Northwest Pennsylvania
Summary Of Significant Accounting Policies [Line Items]
Fair Value													$ 0
Adjustments of the asset retirement liability additional expense										$ 9,800,000	$ 158,000	$ 1,900,000
Other property and equipment | Minimum
Summary Of Significant Accounting Policies [Line Items]
Useful life of assets										3 years
Other property and equipment | Maximum
Summary Of Significant Accounting Policies [Line Items]
Useful life of assets										10 years

[1] Includes capitalized asset retirement costs and the associated accumulated amortization.