Condensed Consolidated Statements of Comprehensive Income (Loss) (Unaudited) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Statement Of Income And Comprehensive Income [Abstract]
Net income (loss)	$ 187,905	$ 2,638	$ 319,011	$ (14,194)
Other comprehensive income (loss), net of tax:
Net loss arising during the period, net of tax benefit of $0, $0, $0, and $6,817				(19,402)
Effect of pension settlement, net of tax benefit of $0, $0, $0 and $11,177				31,811
Amortization of prior service credit included in net income (loss), net of tax benefit of $72, $75, $145 and $152	(205)	(215)	(409)	(430)
Amortization of actuarial loss included in net income (loss), net of tax expense of $1,073, $1,034, $2,182, and $2,223	3,050	2,942	6,207	6,326
Cash flow hedges, net of tax expense (benefit) of $(2,179), $4, $1,853 and $(1,806)	(31,163)	(1,847)	32,944	(40,372)
Other comprehensive income (loss), net of tax	(28,318)	880	38,742	(22,067)
Comprehensive income (loss)	$ 159,587	$ 3,518	$ 357,753	$ (36,261)