Retirement Plans and Other Postretirement Benefits - Components of Net Periodic Benefit Cost (Income) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Retirement Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 124	$ 160	$ 399	$ 501
Interest cost	487	516	1,439	1,553
Expected return on plan assets	(797)	(731)	(2,298)	(2,238)
Amortization of net actuarial loss	309	313	886	958
Accretion of prior service cost	(71)	(71)	(212)	(212)
Net periodic benefit cost	52	187	214	562
Other Post Retirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	12	11	36	34
Interest cost	29	23	88	89
Expected return on plan assets	0	0	0	0
Amortization of net actuarial loss	11	17	32	30
Accretion of prior service cost	0	0	0	0
Net periodic benefit cost	$ 52	$ 51	$ 156	$ 153