Retirement Plans - Periodic Pension Cost (Details) - Pension Plans, Defined Benefit [Member] - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Defined Benefit Plan Disclosure [Line Items]
Interest cost	$ 56	$ 63	$ 112	$ 126
Expected return on assets	(79)	(74)	(158)	(148)
Amortization of net loss	21	24	42	48
Net periodic pension cost	$ (2)	$ 13	$ (4)	$ 26