Retirement Benefits (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($) person	Mar. 31, 2017 USD ($)
Postemployment Benefits [Abstract]
Service cost	$ 215	$ 210
Interest cost	274	284
Expected return on plan assets	(692)	(630)
Amortization of net loss	128	169
Net Periodic Benefit Expense	$ (75)	$ 33
Number of active, vested, terminated and retired persons in the Plan | person	353