Pension and Other Postretirement Plans (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Pension Benefits [Member]
Components of net periodic benefit cost:
Service cost	$ 1,404	$ 1,360	$ 2,861	$ 2,692
Interest cost	3,222	3,194	6,430	6,313
Expected return on assets	(6,552)	(5,740)	(13,120)	(11,482)
Amortization of:
Prior service credit	(11)	(4)	(22)	(8)
Actuarial loss	1,341	1,711	2,639	3,175
Net periodic benefit (credit) cost	(596)	521	(1,212)	690
Postretirement Benefits [Member]
Components of net periodic benefit cost:
Service cost	310	423	642	629
Interest cost	648	912	1,306	1,356
Expected return on assets	(814)	(971)	(1,616)	(1,443)
Amortization of:
Transition obligation	0	1	1	1
Prior service credit	(667)	(898)	(1,334)	(1,334)
Actuarial loss	116	251	244	380
Net periodic benefit (credit) cost	$ (407)	$ (282)	$ (757)	$ (411)