CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Statement Of Income And Comprehensive Income [Abstract]
Net (loss) income	$ (680,048)	$ (27,581)	$ (681,642)	$ 89,023
Postretirement benefits:
Actuarial (loss) gain		(12)	7	(37)
Amortization of prior service costs	92	92	277	276
Income tax expense	(22)	(20)	(70)	(59)
Total comprehensive (loss) income	$ (679,978)	$ (27,521)	$ (681,428)	$ 89,203