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Stock-Based Compensation, Restricted Stock Units (Details) - Restricted Stock Units [Member]
6 Months Ended
Jun. 30, 2017
$ / shares
shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Units granted (in shares) | shares 293,032
Weighted average grant date fair value (in dollars per share) | $ / shares $ 168.77
Vesting period 3 years
Retirement eligibility, employee age 60 years
Service period, minimum 5 years
Retirement eligibility, employee age with no service requirement 65 years
Directors [Member]  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Vesting period 1 year