Pension and Other Postretirement Plans (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Pension Benefits [Member]
Components of net periodic benefit cost:
Service cost	$ 1,431	$ 1,351	$ 4,292	$ 4,043
Interest cost	3,215	3,168	9,645	9,481
Expected return on assets	(6,560)	(5,762)	(19,680)	(17,244)
Amortization of:
Prior service credit	(11)	(4)	(33)	(12)
Actuarial loss	1,319	1,594	3,958	4,769
Net periodic benefit (credit) cost	(606)	347	(1,818)	1,037
Postretirement Benefits [Member]
Components of net periodic benefit cost:
Service cost	320	315	962	944
Interest cost	653	678	1,959	2,034
Expected return on assets	(808)	(722)	(2,424)	(2,165)
Amortization of:
Transition obligation	1	1	2	2
Prior service credit	(667)	(667)	(2,001)	(2,001)
Actuarial loss	122	190	366	570
Net periodic benefit (credit) cost	$ (379)	$ (205)	$ (1,136)	$ (616)