CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Statement Of Income And Comprehensive Income [Abstract]
Net income	$ 27,151	$ 166,195
Postretirement benefits:
Actuarial gain		7
Amortization of prior service costs	92	92
Income tax expense	(22)	(26)
Total comprehensive income	$ 27,221	$ 166,268