Employee Benefit Plans (Schedule of Changes in Accumulated Postretirement Benefits Obligations) (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, Change in Benefit Obligation [Roll Forward]
Obligation at the Beginning of Year	$ 814	$ 783
Unrecognized Loss (Gain)	226	24
Components of Net Periodic Postretirement Benefit Cost
Service Cost	50	41	$ 42
Interest Cost	29	29	26
Net Expected Benefit Payments	(58)	(63)
Obligation at End of Year	$ 1,061	$ 814	$ 783