CONSOLIDATED STATEMENTS OF COMPREHENSIVE (LOSS) INCOME - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Statement Of Income And Comprehensive Income [Abstract]
Net (loss) income	$ (1,716,297)	$ (1,746,481)	$ 333,146
Postretirement benefits:
Actuarial (loss) gain	(532)	526	0
Prior service cost	0	0	(1,769)
Amortization of prior service costs	369	369	0
Income tax benefit (expense)	33	(221)	437
Total comprehensive (loss) income	$ (1,716,427)	$ (1,745,807)	$ 331,814