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Stock Plans - Weighted Average Risk Assumptions (Details) - Performance Stock Unit
3 Months Ended
Mar. 31, 2018
$ / shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Fair value per unit (dollars per share) $ 83.58
Monte Carlo Approach  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Fair value per unit (dollars per share) $ 83.58
Risk-free interest rate 2.36%
Expected dividend yield 1.82%
Expected volatility 28.30%
MSA stock beta 1.240