Pension and Other Post Retirement Benefit Plans - Other Comprehensive Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Retirement Benefits [Abstract]
Net periodic pension cost	$ 654	$ 509	$ 457
Amortization of loss	1,504	(228)	1,299
Other Comprehensive (Income) Loss, Defined Benefit Plan, Amortization Cost (Credit), before Tax	1,979	1,656	1,722
Amortization of prior service cost	90	79	64
Total recognized in other comprehensive income	3,573	1,507	3,085
Total recognized in net periodic pension cost and other comprehensive income	$ 2,919	$ 998	$ 2,628