Retirement Plans and Other Postretirement Benefits - Schedule of Net Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Amortization of:
Portion of cost (credit) charged to expense	$ (6,374)	$ (12,006)	$ (11,488)	$ (24,865)
Pension Benefits
Retirement Plans and Other Benefits
Service cost — benefits earned during the period	12,408	14,121	24,951	28,334
Interest cost on benefit obligation	34,069	31,338	68,421	62,345
Expected return on plan assets	(43,049)	(45,759)	(85,942)	(91,426)
Amortization of:
Prior service credit	0	0	0	0
Net actuarial loss	10,053	8,259	21,292	16,041
Net periodic benefit cost (credit)	13,481	7,959	28,722	15,294
Portion of cost (credit) charged to expense	7,000	2,769	15,244	5,011
Other Benefits
Retirement Plans and Other Benefits
Service cost — benefits earned during the period	4,470	5,445	9,184	10,550
Interest cost on benefit obligation	7,421	6,973	14,947	14,074
Expected return on plan assets	(9,603)	(10,520)	(19,206)	(21,041)
Amortization of:
Prior service credit	(9,455)	(9,461)	(18,910)	(18,921)
Net actuarial loss	0	0	0	0
Net periodic benefit cost (credit)	(7,167)	(7,563)	(13,985)	(15,338)
Portion of cost (credit) charged to expense	$ (5,063)	$ (5,119)	$ (9,880)	$ (10,724)