Retirement Plans and Other Postretirement Benefits - Schedule of Net Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Amortization of:
Portion of cost (credit) charged to expense	$ (13,911)	$ (5,114)
Pension Benefits
Retirement Plans and Other Benefits
Service cost — benefits earned during the period	14,257	12,543
Interest cost on benefit obligation	29,761	34,352
Expected return on plan assets	(46,806)	(42,893)
Amortization of:
Prior service credit	0	0
Net actuarial loss	9,011	11,239
Net periodic benefit cost (credit)	6,223	15,241
Portion of cost (credit) charged to expense	1,342	8,244
Other Benefits
Retirement Plans and Other Benefits
Service cost — benefits earned during the period	5,717	4,714
Interest cost on benefit obligation	6,512	7,526
Expected return on plan assets	(10,019)	(9,603)
Amortization of:
Prior service credit	(9,394)	(9,455)
Net actuarial loss	0	0
Net periodic benefit cost (credit)	(7,184)	(6,818)
Portion of cost (credit) charged to expense	$ (5,456)	$ (4,817)