Retirement Plans and Other Benefits - Changes Benefit Obligations and Funded Status (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Pension Benefits
Change in Benefit Obligation
Benefit obligation at the beginning of the period	$ 3,190,626	$ 3,394,186
Service cost	49,902	56,669	$ 54,858
Interest cost	136,843	124,689	129,756
Benefit payments	(177,882)	(184,161)
Actuarial (gain) loss	413,625	(200,757)
Benefit obligation at the end of the period	3,613,114	3,190,626	3,394,186
Change in Plan Assets
Balance at the beginning of the period	2,733,476	3,057,027
Actual return on plan assets	602,030	(201,078)
Employer contributions	150,000	50,000	100,000
Benefit payments	(167,155)	(172,473)
Transfer to active union medical account	0	0
Balance at the end of the period	3,318,351	2,733,476	3,057,027
Funded Status at the end of the period	(294,763)	(457,150)
Other Benefits
Change in Benefit Obligation
Benefit obligation at the beginning of the period	676,771	753,393
Service cost	18,369	21,100	17,119
Interest cost	29,894	28,147	29,959
Benefit payments	(32,486)	(31,540)
Actuarial (gain) loss	54,376	(94,329)
Benefit obligation at the end of the period	746,924	676,771	753,393
Change in Plan Assets
Balance at the beginning of the period	723,677	1,022,371
Actual return on plan assets	144,095	(40,354)
Employer contributions	0	0
Benefit payments	(30,278)	(72,453)
Transfer to active union medical account	0	(185,887)
Balance at the end of the period	837,494	723,677	$ 1,022,371
Funded Status at the end of the period	$ 90,570	$ 46,906