Pension and Other Postretirement Plans (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Pension Benefits [Member]
Components of net periodic benefit cost:
Service cost	$ 1,351	$ 1,413	$ 4,043	$ 4,358
Interest cost	3,168	3,161	9,481	9,744
Expected return on assets	(5,762)	(5,724)	(17,244)	(17,646)
Amortization of:
Prior service (credit) cost	(4)	3	(12)	8
Actuarial loss	1,594	1,388	4,769	4,278
Net periodic benefit cost (credit)	347	241	1,037	742
Postretirement Benefits [Member]
Components of net periodic benefit cost:
Service cost	315	361	944	1,084
Interest cost	678	753	2,034	2,258
Expected return on assets	(722)	(787)	(2,165)	(2,360)
Amortization of:
Transition obligation	1	1	2	3
Prior service (credit) cost	(667)	(744)	(2,001)	(2,233)
Actuarial loss	190	178	570	533
Net periodic benefit cost (credit)	$ (205)	$ (238)	$ (616)	$ (715)