Retirement Plans and Other Postretirement Benefits - Changes Benefit Obligations and Funded Status (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Pension Benefits
Change in Benefit Obligation
Benefit obligation at the beginning of the period	$ 3,613,114	$ 3,190,626
Service cost	56,233	49,902	$ 56,669
Interest cost	118,567	136,843	124,689
Benefit payments	(191,704)	(177,882)
Actuarial (gain) loss	306,657	413,625
Benefit obligation at the end of the period	3,902,867	3,613,114	3,190,626
Change in Plan Assets
Balance at the beginning of the period	3,318,351	2,733,476
Actual return on plan assets	642,373	602,030
Employer contributions	100,000	150,000	50,000
Benefit payments	(174,180)	(167,155)
Balance at the end of the period	3,886,544	3,318,351	2,733,476
Funded Status at the end of the period	(16,323)	(294,763)
Other Benefits
Change in Benefit Obligation
Benefit obligation at the beginning of the period	746,924	676,771
Service cost	22,236	18,369	21,100
Interest cost	25,857	29,894	28,147
Benefit payments	(31,511)	(32,486)
Actuarial (gain) loss	(139,472)	54,376
Benefit obligation at the end of the period	624,034	746,924	676,771
Change in Plan Assets
Balance at the beginning of the period	837,494	723,677
Actual return on plan assets	150,076	144,095
Employer contributions	0	0
Benefit payments	(26,405)	(30,278)
Balance at the end of the period	961,165	837,494	$ 723,677
Funded Status at the end of the period	$ 337,131	$ 90,570