Retirement Plans and Other Postretirement Benefits - Schedule of Net Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Amortization of:
Portion of cost (credit) charged to expense	$ (5,114)	$ (12,859)
Pension Benefits
Retirement Plans and Other Benefits
Service cost — benefits earned during the period	12,543	14,213
Interest cost on benefit obligation	34,352	31,007
Expected return on plan assets	(42,893)	(45,667)
Amortization of:
Prior service cost (credit)	0	0
Net actuarial loss	11,239	7,782
Net periodic benefit cost (credit)	15,241	7,335
Portion of cost (credit) charged to expense	8,244	2,242
Other Benefits
Retirement Plans and Other Benefits
Service cost — benefits earned during the period	4,714	5,105
Interest cost on benefit obligation	7,526	7,101
Expected return on plan assets	(9,603)	(10,520)
Amortization of:
Prior service cost (credit)	(9,455)	(9,461)
Net actuarial loss	0	0
Net periodic benefit cost (credit)	(6,818)	(7,775)
Portion of cost (credit) charged to expense	$ (4,817)	$ (5,605)