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, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Retirement Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 110	$ 140	$ 219	$ 280
Interest cost	476	439	952	880
Expected return on plan assets	(770)	(795)	(1,541)	(1,591)
Amortization of net actuarial loss	294	281	588	561
Accretion of prior service cost	(46)	(71)	(91)	(142)
Net periodic benefit cost (income)	64	(6)	127	(12)
Other Post Retirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	16	22	33	44
Interest cost	24	23	47	46
Expected return on plan assets	0	0	0	0
Amortization of net actuarial loss	0	1	0	2
Accretion of prior service cost	0	0	0	0
Net periodic benefit cost (income)	$ 40	$ 46	$ 80	$ 92