Summary of Significant Accounting Policies - Additional Information 1 (Detail) - USD ($)		3 Months Ended								12 Months Ended
		Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Summary Of Significant Accounting Policies [Line Items]
Net book value of unproved properties	[1]	$ 2,644,000,000				$ 2,923,803,000				$ 2,644,000,000	$ 2,923,803,000	$ 949,155,000
Abandonment and impairment of unproved properties		217,544,000	$ 42,568,000	$ 5,193,000	$ 4,420,000	6,307,000	$ 6,082,000	$ 7,059,000	$ 10,628,000	269,725,000	30,076,000	47,619,000
Depreciation expense										7,700,000	8,400,000	11,900,000
Marketable securities held in deferred compensation plans		67,100,000				61,700,000				67,100,000	61,700,000
Surface acreage		$ 9,600,000				$ 10,600,000				9,600,000	10,600,000
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										$ 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.