CONSOLIDATED STATEMENTS OF COMPREHENSIVE LOSS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Statement Of Income And Comprehensive Income [Abstract]
Net loss	$ (711,777)	$ (1,716,297)	$ (1,746,481)
Postretirement benefits:
Actuarial gain (loss)	39	(532)	526
Amortization of prior service costs	369	369	369
Income tax (expense) benefit	(99)	33	(221)
Total comprehensive loss	$ (711,468)	$ (1,716,427)	$ (1,745,807)