Condensed Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Statement Of Income And Comprehensive Income [Abstract]
Net income (loss)	$ 65,667	$ 81,007	$ 384,678	$ 66,813
Other comprehensive income (loss), net of tax:
Net loss arising during the period, net of tax benefit of $0, $0, $0, and $6,817	0	0	0	(19,402)
Effect of pension settlement, net of tax benefit of $0, $0, $0 and $11,177	0	0	0	31,811
Amortization of prior service credit included in net income, net of tax benefit of $73, $75, $217 and $227	(204)	(215)	(614)	(645)
Amortization of actuarial loss included in net income, net of tax expense of $1,073, $1,111, $3,254, and $3,334	3,050	3,163	9,258	9,489
Cash flow hedges, net of tax expense (benefit) of $397, $763, $2,249 and $(1,043)	6,636	11,332	39,580	(29,040)
Other comprehensive income (loss), net of tax	9,482	14,280	48,224	(7,787)
Comprehensive income (loss)	$ 75,149	$ 95,287	$ 432,902	$ 59,026