Retirement Plans and Other Postretirement Benefits - Components of Net Periodic Benefit Cost (Income) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Retirement Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 151	$ 171
Interest cost	465	518
Expected return on plan assets	(753)	(754)
Amortization of net actuarial loss	288	323
Accretion of prior service cost	(70)	(71)
Net periodic benefit cost	81	187
Other Post Retirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	12	12
Interest cost	29	33
Expected return on plan assets	0	0
Amortization of net actuarial loss	11	6
Accretion of prior service cost	0	0
Net periodic benefit cost	$ 52	$ 51