Retirement Plans and Other Postretirement Benefits - Schedule of Net Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Amortization of:
Portion of cost/(benefit) charged to expense	$ (28,135)	$ (14,118)	$ (84,101)	$ (42,171)
Pension Benefits
Retirement Plans and Other Benefits
Service cost — benefits earned during the period	15,309	14,058	45,927	42,174
Interest cost on benefit obligation	24,641	29,642	73,924	88,925
Expected return on plan assets	(50,657)	(46,861)	(151,971)	(140,582)
Amortization of:
Prior service credit	0	0	0	0
Net actuarial loss (gain)	3,987	8,653	11,961	25,959
Net periodic benefit cost/(benefit)	(6,720)	5,492	(20,159)	16,476
Portion of cost/(benefit) charged to expense	(7,803)	736	(24,428)	2,349
Other Benefits
Retirement Plans and Other Benefits
Service cost — benefits earned during the period	4,449	5,559	13,347	16,677
Interest cost on benefit obligation	4,128	6,464	12,385	19,393
Expected return on plan assets	(10,361)	(10,019)	(31,083)	(30,057)
Amortization of:
Prior service credit	(9,425)	(9,394)	(28,279)	(28,182)
Net actuarial loss (gain)	(2,523)	0	(7,569)	0
Net periodic benefit cost/(benefit)	(13,732)	(7,390)	(41,199)	(22,169)
Portion of cost/(benefit) charged to expense	$ (9,765)	$ (5,286)	$ (28,901)	$ (15,798)