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Other Employee Benefit Plans - Shares purchased (Details) - ESPP [Member] - $ / shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Shares purchased 4,718 4,988 5,136
Average price per share $ 77.05 $ 46.46 $ 49.10