Defined Benefit Post-retirement Plans (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Defined Benefit Post-retirement Plans [Abstract]
Employer contributions	$ 0	$ 0
Pension Benefits [Member]
Components of net periodic (benefit) cost [Abstract]
Service cost	420	402
Interest cost	920	1,042
Expected return on plan assets	(2,123)	(1,985)
Net amortization	251	415
Total net periodic (benefit) cost	(532)	(126)
Other Benefits [Member]
Components of net periodic (benefit) cost [Abstract]
Service cost	3	3
Interest cost	82	86
Expected return on plan assets	0	0
Net amortization	44	20
Total net periodic (benefit) cost	$ 129	$ 109