Income Taxes - Additional Information (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
Income Taxes [Line Items]
Income tax benefit	$ (68,784,000)	$ 24,137,000	$ (28,518,000)	$ 42,676,000	$ (349,080,000)	$ (71,992,000)	$ 57,651,000	$ 112,395,000	$ (30,489,000)	$ (251,026,000)	$ (280,750,000)
Corporate tax rate									21.00%	35.00%	35.00%
Deferred tax liabilities	666,668,000				$ 693,356,000				$ 666,668,000	$ 693,356,000
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, 2019
Maximum
Income Taxes [Line Items]
Period during which net operating loss carryforwards and alternative minimum tax expire									Dec. 31, 2037
Amendment on Internal Revenue Code of 1986
Income Taxes [Line Items]
Corporate tax rate									21.00%	35.00%
One-time tax benefit related to tax law changes										$ 334,000,000
Louisiana, Oklahoma, Pennsylvania, Texas and West Virginia
Income Taxes [Line Items]
Valuation allowances	101,400,000								$ 101,400,000
Federal
Income Taxes [Line Items]
Valuation allowances	19,000,000								19,000,000
Federal and State
Income Taxes [Line Items]
Net operating loss carryforwards	2,000,000,000								2,000,000,000
Unexpired net operating loss carryforwards	520,100,000								520,100,000
Pennsylvania
Income Taxes [Line Items]
Net operating loss carryforwards	882,700,000								$ 882,700,000
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, 2038
Louisiana
Income Taxes [Line Items]
Net operating loss carryforwards	$ 338,600,000								$ 338,600,000
Louisiana | Minimum
Income Taxes [Line Items]
Period during which net operating loss carryforwards and alternative minimum tax expire									Dec. 31, 2034
Louisiana | Maximum
Income Taxes [Line Items]
Period during which net operating loss carryforwards and alternative minimum tax expire									Dec. 31, 2038