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Other Benefit Plans (Schedule of Employees Eligible to Participate in Employee Stock Purchase Plan) (Details) - $ / shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Defined Benefit Pension Plans And Defined Benefit Postretirement Plans Disclosure [Abstract]      
ESPP shares purchased 3,578 4,465 4,162
Average purchase price per share $ 29.027 $ 18.520 $ 15.865