Condensed 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 income	$ 81,007	$ 20,565	$ 66,813	$ 44,262
Other comprehensive income (loss), net of tax:
Net loss arising during the period, net of tax benefit of $0, $0, $6,817 and $0			(19,402)
Effect of pension settlement, net of tax benefit of $0, $0, $11,177 and $0			31,811
Amortization of prior service credit included in net income, net of tax benefit of $75, $562, $227 and $1,684	(215)	(1,598)	(645)	(4,792)
Amortization of actuarial loss included in net income, net of tax expense of $1,111, $943, $3,334 and $2,829	3,163	2,685	9,489	8,053
Cash flow hedges, net of tax expense (benefit) of $763, $(387), $(1,043) and $(1,300)	11,332	(6,978)	(29,040)	(25,908)
Other comprehensive income (loss), net of tax	14,280	(5,891)	(7,787)	(22,647)
Comprehensive income	$ 95,287	$ 14,674	$ 59,026	$ 21,615