Retirement Plans - Accumulated Other Comprehensive loss (Details 4) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Accumulated Other Comprehensive Loss
Amortization period	18 years
Post-Retirement Life Insurance Plan
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Accumulated Benefit Obligation	$ 5,134	$ 4,952
Accumulated Other Comprehensive Loss
Amortization period	3 years
Post-Retirement Life Insurance Plan | Accumulated Defined Benefit Plans Adjustment, Net Unamortized Gain (Loss)
Accumulated Other Comprehensive Loss
Beginning balance	$ (560)	(669)
Amortization of retirement benefits, net of tax	64	95
Net actuarial (loss) gain	117	14
Ending balance	(379)	(560)
Domestic Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Accumulated Benefit Obligation	228,934	247,276
Accumulated Other Comprehensive Loss
Beginning balance	89,763	96,388
Amortization of retirement benefits, net of tax	(3,685)	(3,817)
Settlements	(8,585)
Net actuarial (loss) gain	(1,753)	(2,808)
Foreign exchange impact	0	0
Ending balance	75,740	89,763
Domestic Plan [Member] | Accumulated Defined Benefit Plans Adjustment, Net Unamortized Gain (Loss)
Accumulated Other Comprehensive Loss
Beginning balance	89,763	96,388
Amortization of retirement benefits, net of tax	(3,685)	(3,817)
Settlements	(8,585)
Net actuarial (loss) gain	(1,753)	(2,808)
Foreign exchange impact	0	0
Ending balance	75,740	89,763
Domestic Plan [Member] | Accumulated Defined Benefit Plans Adjustment, Net Prior Service Cost (Credit)
Accumulated Other Comprehensive Loss
Beginning balance	0	0
Amortization of retirement benefits, net of tax	0	0
Settlements	0
Net actuarial (loss) gain	0	0
Foreign exchange impact	0	0
Ending balance	0	0
Foreign Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Accumulated Benefit Obligation	2,535	2,295
Foreign Plan [Member] | Accumulated Defined Benefit Plans Adjustment, Net Unamortized Gain (Loss)
Accumulated Other Comprehensive Loss
Beginning balance	1,743	1,639
Amortization of retirement benefits, net of tax	10	85
Settlements	0
Net actuarial (loss) gain	2	12
Foreign exchange impact	143	7
Ending balance	$ 1,898	$ 1,743