Pension and Other Postretirement Plans (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Pension Benefits [Member]
Components of net periodic benefit cost:
Service cost	$ 1,360	$ 1,564	$ 2,692	$ 2,945
Interest cost	3,194	3,494	6,313	6,583
Expected return on assets	(5,740)	(6,329)	(11,482)	(11,922)
Amortization of:
Prior service (credit) cost	(4)	3	(8)	5
Actuarial loss	1,711	1,534	3,175	2,890
Net periodic benefit cost (credit)	521	266	690	501
Postretirement Benefits [Member]
Components of net periodic benefit cost:
Service cost	423	399	629	723
Interest cost	912	831	1,356	1,505
Expected return on assets	(971)	(868)	(1,443)	(1,573)
Amortization of:
Transition obligation	1	1	1	2
Prior service (credit) cost	(898)	(822)	(1,334)	(1,489)
Actuarial loss	251	196	380	355
Net periodic benefit cost (credit)	$ (282)	$ (263)	$ (411)	$ (477)