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, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Retirement Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 117	$ 109	$ 350	$ 328
Interest cost	425	476	1,259	1,428
Expected loss on plan assets	(816)	(772)	(2,450)	(2,313)
Amortization of net actuarial loss	300	294	882	882
Accretion of prior service cost	0	(45)	0	(136)
Net periodic benefit cost	26	62	41	189
Other Post Retirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	27	17	82	50
Interest cost	23	23	72	70
Expected loss on plan assets	0	0	0	0
Amortization of net actuarial loss	7	0	19	0
Accretion of prior service cost	0	0	0	0
Net periodic benefit cost	$ 57	$ 40	$ 173	$ 120