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, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Retirement Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 124	$ 170	$ 275	$ 341
Interest cost	487	519	952	1,037
Expected return on plan assets	(748)	(753)	(1,501)	(1,507)
Amortization of net actuarial loss	289	322	577	645
Accretion of prior service cost	(71)	(70)	(141)	(141)
Net periodic benefit cost	81	188	162	375
Other Post Retirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	12	11	24	23
Interest cost	30	33	59	66
Expected return on plan assets	0	0	0	0
Amortization of net actuarial loss	10	7	21	13
Accretion of prior service cost	0	0	0	0
Net periodic benefit cost	$ 52	$ 51	$ 104	$ 102