Retirement Plans and Other Postretirement Benefits - Components of Net Periodic Benefit Cost (Income) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Retirement Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 140	$ 124	$ 280	$ 275
Interest cost	439	487	880	952
Expected return on plan assets	(795)	(748)	(1,591)	(1,501)
Amortization of net actuarial loss	281	289	561	577
Accretion of prior service cost	(71)	(71)	(142)	(141)
Net periodic benefit (income) cost	(6)	81	(12)	162
Other Post Retirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	22	12	44	24
Interest cost	23	30	46	59
Expected return on plan assets	0	0	0	0
Amortization of net actuarial loss	1	10	2	21
Accretion of prior service cost	0	0	0	0
Net periodic benefit (income) cost	$ 46	$ 52	$ 92	$ 104