Defined Benefit Post-retirement Plans (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Defined Benefit Post-retirement Plans [Abstract]
Employer contributions			$ 0
Pension Benefits [Member]
Components of net periodic (benefit) cost [Abstract]
Service cost	$ 402	$ 560	804	$ 1,120
Interest cost	1,042	1,051	2,084	2,102
Expected return on plan assets	(1,985)	(1,835)	(3,970)	(3,670)
Net amortization	415	483	831	966
Total (benefit) cost	(126)	259	(251)	518
Other Benefits [Member]
Components of net periodic (benefit) cost [Abstract]
Service cost	3	4	6	8
Interest cost	86	94	172	188
Expected return on plan assets	0	0	0	0
Net amortization	20	29	40	58
Total (benefit) cost	$ 109	$ 127	$ 218	$ 254