Retirement Plans and Other Postretirement Benefits - Schedule of Net Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Amortization of:
Portion of cost/(benefit) charged to expense	$ (28,175)	$ (14,142)	$ (55,966)	$ (28,053)
Pension Benefits
Retirement Plans and Other Benefits
Service cost — benefits earned during the period	14,939	13,859	30,618	28,116
Interest cost on benefit obligation	24,614	29,522	49,283	59,283
Expected return on plan assets	(50,706)	(46,915)	(101,314)	(93,721)
Amortization of:
Prior service credit	0	0	0	0
Net actuarial loss (gain)	3,989	8,295	7,974	17,306
Net periodic benefit cost/(benefit)	(7,164)	4,761	(13,439)	10,984
Portion of cost/(benefit) charged to expense	(8,614)	271	(16,625)	1,613
Other Benefits
Retirement Plans and Other Benefits
Service cost — benefits earned during the period	4,341	5,401	8,898	11,118
Interest cost on benefit obligation	4,095	6,417	8,257	12,929
Expected return on plan assets	(10,361)	(10,019)	(20,722)	(20,038)
Amortization of:
Prior service credit	(9,427)	(9,394)	(18,854)	(18,788)
Net actuarial loss (gain)	(2,641)	0	(5,046)	0
Net periodic benefit cost/(benefit)	(13,993)	(7,595)	(27,467)	(14,779)
Portion of cost/(benefit) charged to expense	$ (9,608)	$ (5,056)	$ (19,136)	$ (10,512)