Summary of Principal Components of Net Deferred Income Tax Liability (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets:
Property and equipment	$ 37,345	$ 183,303
Pension and postretirement benefits	452	710
Share-based compensation	435	28
Net operating loss (“NOL”) carryforwards	127,821	87,622
Deferred Tax Assets, Derivative Instruments	8,752	9,579
Other	7,608	7,166
Deferred tax assets, gross	182,413	288,408
Less: Valuation allowance	(177,470)	(288,408)
Net deferred tax assets	$ 4,943	$ 0