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Stock Plans - Weighted Average Risk Assumptions (Details)
6 Months Ended
Jun. 30, 2019
$ / shares
Performance Stock Unit  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Fair value per unit (dollars per share) $ 100.77
Monte Carlo Approach | Stock options exercised  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Fair value per unit (dollars per share) $ 59.07
Risk-free interest rate 2.30%
Expected dividend yield 1.70%
Expected volatility 31.00%
Expected life (years) 6 years 4 months 24 days
Monte Carlo Approach | Performance Stock Unit  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Fair value per unit (dollars per share) $ 99.82
Risk-free interest rate 2.47%
Expected dividend yield 1.57%
Expected volatility 26.60%
MSA stock beta 1.094