Retirement Plans and Other Benefits - Changes Benefit Obligations and Funded Status (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Pension Benefits
Change in Benefit Obligation
Benefit obligation at the beginning of the period	$ 3,394,186	$ 3,204,462
Service cost	56,669	54,858	$ 53,792
Interest cost	124,689	129,756	131,647
Benefit payments	(184,161)	(166,342)
Actuarial (gain) loss	(200,757)	171,452
Benefit obligation at the end of the period	3,190,626	3,394,186	3,204,462
Change in Plan Assets
Balance at the beginning of the period	3,057,027	2,675,357
Actual return on plan assets	(201,078)	428,374
Employer contributions	50,000	100,000	100,000
Benefit payments	(172,473)	(146,704)
Transfer to active union medical account	0	0
Balance at the end of the period	2,733,476	3,057,027	2,675,357
Funded Status at the end of the period	(457,150)	(337,159)
Other Benefits
Change in Benefit Obligation
Benefit obligation at the beginning of the period	753,393	716,445
Service cost	21,100	17,119	14,993
Interest cost	28,147	29,959	29,721
Benefit payments	(31,540)	(30,144)
Actuarial (gain) loss	(94,329)	20,014
Benefit obligation at the end of the period	676,771	753,393	716,445
Change in Plan Assets
Balance at the beginning of the period	1,022,371	882,651
Actual return on plan assets	(40,354)	139,367
Employer contributions	0	353
Benefit payments	(72,453)	0
Transfer to active union medical account	(185,887)	0
Balance at the end of the period	723,677	1,022,371	$ 882,651
Funded Status at the end of the period	$ 46,906	$ 268,978