Retirement Plans and Other Postretirement Benefits - Schedule of Net Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Amortization of:
Portion of cost/(benefit) charged to expense	$ (27,791)	$ (13,911)
Pension Benefits
Retirement Plans and Other Benefits
Service cost — benefits earned during the period	15,679	14,257
Interest cost on benefit obligation	24,669	29,761
Expected return on plan assets	(50,608)	(46,806)
Amortization of:
Prior service credit	0	0
Net actuarial loss (gain)	3,985	9,011
Net periodic benefit cost/(benefit)	(6,275)	6,223
Portion of cost/(benefit) charged to expense	(8,011)	1,342
Other Benefits
Retirement Plans and Other Benefits
Service cost — benefits earned during the period	4,557	5,717
Interest cost on benefit obligation	4,162	6,512
Expected return on plan assets	(10,361)	(10,019)
Amortization of:
Prior service credit	(9,427)	(9,394)
Net actuarial loss (gain)	(2,405)	0
Net periodic benefit cost/(benefit)	(13,474)	(7,184)
Portion of cost/(benefit) charged to expense	$ (9,528)	$ (5,456)