Retirement Plans and Other Postretirement Benefits - Net Periodic Benefit Costs and Portion including Portion Charged to Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Net periodic benefit costs and the portion of these costs charged to expense
Portion of cost/(benefit) charged to expense	$ (56,341)	$ (22,989)	$ (49,791)
Pension Benefits
Net periodic benefit costs and the portion of these costs charged to expense
Service cost-benefits earned during the period	56,233	49,902	56,669
Interest cost on benefit obligation	118,567	136,843	124,689
Expected return on plan assets	(187,443)	(171,884)	(182,853)
Prior service credit	0	0	0
Net actuarial loss	34,612	42,584	32,082
Net periodic benefit cost/(benefit)	21,969	57,445	30,587
Portion of cost/(benefit) charged to expense	3,386	30,312	10,120
Other Benefits
Net periodic benefit costs and the portion of these costs charged to expense
Service cost-benefits earned during the period	22,236	18,369	21,100
Interest cost on benefit obligation	25,857	29,894	28,147
Expected return on plan assets	(40,077)	(38,412)	(42,082)
Prior service credit	(37,575)	(37,821)	(37,842)
Net actuarial loss	0	0	0
Net periodic benefit cost/(benefit)	(29,559)	(27,970)	(30,677)
Portion of cost/(benefit) charged to expense	$ (20,966)	$ (19,859)	$ (21,426)