Retirement Plans and Other Postretirement Benefits - Schedule of Net Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Amortization of:
Portion of cost (credit) charged to expense	$ (14,142)	$ (6,374)	$ (28,053)	$ (11,488)
Pension Benefits
Retirement Plans and Other Benefits
Service cost — benefits earned during the period	13,859	12,408	28,116	24,951
Interest cost on benefit obligation	29,522	34,069	59,283	68,421
Expected return on plan assets	(46,915)	(43,049)	(93,721)	(85,942)
Amortization of:
Prior service credit	0	0	0	0
Net actuarial loss	8,295	10,053	17,306	21,292
Net periodic benefit cost (credit)	4,761	13,481	10,984	28,722
Portion of cost (credit) charged to expense	271	7,000	1,613	15,244
Other Benefits
Retirement Plans and Other Benefits
Service cost — benefits earned during the period	5,401	4,470	11,118	9,184
Interest cost on benefit obligation	6,417	7,421	12,929	14,947
Expected return on plan assets	(10,019)	(9,603)	(20,038)	(19,206)
Amortization of:
Prior service credit	(9,394)	(9,455)	(18,788)	(18,910)
Net actuarial loss	0	0	0	0
Net periodic benefit cost (credit)	(7,595)	(7,167)	(14,779)	(13,985)
Portion of cost (credit) charged to expense	$ (5,056)	$ (5,063)	$ (10,512)	$ (9,880)