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Stock Plans - Weighted Average Risk Assumptions (Details) - Performance Stock Unit
3 Months Ended 6 Months Ended
Mar. 31, 2017
$ / shares
Jun. 30, 2017
$ / shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Fair value per unit (dollars per share)   $ 72.73
Monte Carlo Approach    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Fair value per unit (dollars per share) $ 72.28  
Risk-free interest rate 1.45%  
Expected dividend yield 2.31%  
Expected volatility 29.10%  
MSA stock beta 1.272