Defined Benefit Post-Retirement Plans (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Defined Benefit Post-Retirement Plans [Abstract]
Employer contributions	$ 0	$ 0	$ 0	$ 0
Pension Benefits [Member]
Components of net periodic (benefit) cost [Abstract]
Service cost	415	329	1,256	1,133
Interest cost	912	1,043	2,752	3,127
Expected return on plan assets	(2,119)	(1,976)	(6,364)	(5,946)
Net amortization	242	454	745	1,284
Total net periodic (benefit) cost	(550)	(150)	(1,611)	(402)
Other Benefits [Member]
Components of net periodic (benefit) cost [Abstract]
Service cost	3	3	8	9
Interest cost	82	88	245	259
Expected return on plan assets	0	0	0	0
Net amortization	44	26	131	66
Total net periodic (benefit) cost	$ 129	$ 117	$ 384	$ 334