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, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Retirement Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 170	$ 193	$ 341	$ 386
Interest cost	519	488	1,037	976
Expected return on plan assets	(753)	(756)	(1,507)	(1,512)
Amortization of net actuarial loss	322	326	645	654
Accretion of prior service cost	(70)	(70)	(141)	(140)
Net periodic benefit cost	188	181	375	364
Other Post Retirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	11	14	23	29
Interest cost	33	27	66	55
Expected return on plan assets	0	0	0	0
Amortization of net actuarial loss	7	14	13	27
Accretion of prior service cost	0	0	0	0
Net periodic benefit cost	$ 51	$ 55	$ 102	$ 111