Employee Benefit Plans (Schedule of Assumptions used to Determine Net Periodic Cost and Benefit Obligations) (Details)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Postretirement Medical and Life Benefit Plan
Defined Benefit Plan Disclosure [Line Items]
Discount Rate	3.35%	3.72%	3.82%