CONSOLIDATED STATEMENTS OF COMPREHENSIVE (LOSS) INCOME - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Statement Of Income And Comprehensive Income [Abstract]
Net (loss) income	$ (1,746,481)	$ 333,146	$ (521,388)
Postretirement benefits:
Actuarial gain	526
Prior service cost		(1,769)
Amortization of prior service costs	369
Income tax (expense) benefit	(221)	437
Total comprehensive (loss) income	$ (1,745,807)	$ 331,814	$ (521,388)