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, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Retirement Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 140	$ 124	$ 420	$ 399
Interest cost	440	487	1,320	1,439
Expected return on plan assets	(849)	(797)	(2,440)	(2,298)
Amortization of net actuarial loss	280	309	841	886
Accretion of prior service cost	(70)	(71)	(212)	(212)
Net periodic benefit (income) cost	(59)	52	(71)	214
Other Post Retirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	22	12	66	36
Interest cost	23	29	69	88
Expected return on plan assets	0	0	0	0
Amortization of net actuarial loss	1	11	3	32
Accretion of prior service cost	0	0	0	0
Net periodic benefit (income) cost	$ 46	$ 52	$ 138	$ 156