Income Taxes - Additional Information (Detail) - USD ($)			3 Months Ended								12 Months Ended
	Dec. 22, 2017	Oct. 18, 2017	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, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Taxes [Line Items]
Income tax benefit			$ (349,080,000)	$ (71,992,000)	$ 57,651,000	$ 112,395,000	$ (95,581,000)	$ (13,705,000)	$ (129,488,000)	$ (41,976,000)		$ (251,026,000)	$ (280,750,000)	$ (338,677,000)
Corporate tax rate												35.00%	35.00%	35.00%
Deferred tax liabilities			693,356,000				$ 943,343,000					$ 693,356,000	$ 943,343,000
Deductible limit			1,000,000									1,000,000
Accrued interest or penalties related to tax amounts			0									$ 0
Minimum
Income Taxes [Line Items]
Period during which net operating loss carryforwards and alternative minimum tax expire												Dec. 31, 2018
Maximum
Income Taxes [Line Items]
Period during which net operating loss carryforwards and alternative minimum tax expire												Dec. 31, 2035
Deferred compensation plan
Income Taxes [Line Items]
Valuation allowances			1,900,000									$ 1,900,000
Amendment on Internal Revenue Code of 1986
Income Taxes [Line Items]
Corporate tax rate												35.00%
One-time tax benefit related to tax law changes	$ 334,000,000
Amendment on Internal Revenue Code of 1986 | Scenario, Forecast
Income Taxes [Line Items]
Corporate tax rate											21.00%
Oklahoma, Texas and West Virginia
Income Taxes [Line Items]
State valuation allowance			36,300,000									$ 36,300,000
Pennsylvania
Income Taxes [Line Items]
State valuation allowance			57,500,000									$ 57,500,000
Minimum net operating loss deduction amount		$ 3,000,000
Percentage of taxable income		30.00%
Net operating loss deduction, description												The Supreme Court ruled that the net operating loss deduction limitation violated the Uniformity Clause of the Pennsylvania Constitution and struck the $3.0 million flat cap limitation but not the percentage of taxable income limitation.
Net operating loss carryforwards			872,600,000									$ 872,600,000
Pennsylvania | Minimum
Income Taxes [Line Items]
Period during which net operating loss carryforwards and alternative minimum tax expire												Dec. 31, 2025
Pennsylvania | Maximum
Income Taxes [Line Items]
Period during which net operating loss carryforwards and alternative minimum tax expire												Dec. 31, 2036
Federal and State
Income Taxes [Line Items]
Net operating loss carryforwards			$ 1,500,000,000									$ 1,500,000,000