Other Postretirement Benefits (Components of Net Periodic Postretirement Benefit Cost) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Interest cost	$ 46	$ 59	$ 66
Amortization of unrecognized prior service cost	2	6	14
Loss amortization (2)	(19)	(13)	0
Net periodic pension cost	$ 29	$ 52	$ 80
Discount rate	3.55%	3.81%	3.94%
Ultimate healthcare trend rates	4.00%
Other Postretirement Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	$ 17	$ 22	$ 26
Interest cost	269	272	300
Amortization of unrecognized prior service cost	44	44	44
Loss amortization (2)	146	134	197
Net periodic pension cost	$ 476	$ 472	$ 567
Discount rate	3.40%	3.80%	3.90%
Annual rate of increase in the per capita cost of: Medical costs before age 65	7.30%	7.50%	7.80%
Annual rate of increase in the per capita cost of: Medical costs after age 65	6.30%	6.50%	6.80%
Annual rate of increase in the per capita cost of: Prescription drug costs	10.50%	10.50%	11.00%