Employee Benefit Plans (Components of Postretirement Benefit Expense) (Details) - Postretirement Medical and Life Benefit Plan - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Service Cost	$ 50	$ 41	$ 42
Interest Cost	29	29	26
Amortization of Unrecognized Net (Gain) Loss	8	6	5
Net Postretirement Benefit Expense	87	76	73
Net (Gain) Loss During Period Recognized in Other Comprehensive Income (Loss)	218	18	17
Total Recognized in Net Postretirement Benefit Expense and Other Comprehensive Income	$ 305	$ 94	$ 90