Retirement Plans and Other Postretirement Benefits - Schedule of Net Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Amortization of:
Portion of cost (credit) charged to expense	$ (12,006)	$ (6,972)	$ (24,865)	$ (13,067)
Pension Benefits
Retirement Plans and Other Benefits
Service cost — benefits earned during the period	14,121	13,669	28,334	27,429
Interest cost on benefit obligation	31,338	32,177	62,345	64,878
Expected return on plan assets	(45,759)	(43,425)	(91,426)	(87,135)
Amortization of:
Prior service cost (credit)	0	20	0	41
Net actuarial loss	8,259	11,460	16,041	23,950
Net periodic benefit cost (credit)	7,959	13,901	15,294	29,163
Portion of cost (credit) charged to expense	2,769	6,894	5,011	14,461
Other Benefits
Retirement Plans and Other Benefits
Service cost — benefits earned during the period	5,445	4,201	10,550	8,559
Interest cost on benefit obligation	6,973	7,415	14,074	14,980
Expected return on plan assets	(10,520)	(13,350)	(21,041)	(26,701)
Amortization of:
Prior service cost (credit)	(9,461)	(9,461)	(18,921)	(18,921)
Net actuarial loss	0	1,104	0	2,559
Net periodic benefit cost (credit)	(7,563)	(10,091)	(15,338)	(19,524)
Portion of cost (credit) charged to expense	$ (5,119)	$ (5,004)	$ (10,724)	$ (9,682)