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Stock-Based Compensation Long-Term Performance Restricted Stock Unit Award Monte Carlo Pricing Model Assumptions (Details) - Restricted Stock Units (RSUs) [Member] - $ / shares
3 Months Ended 9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Weighted average fair value     $ 65.54 $ 41.58
Long Term Performance Restricted Stock Award [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Expected volatility 27.90% 28.80% 28.00% 30.00%
Risk-free interest rate 1.40% 0.80% 1.00% 0.70%
Expected term (years) 2 years 4 months 1 day 2 years 4 months 1 day 1 year 8 months 10 days 1 year 9 months 18 days
Weighted average fair value $ 80.64 $ 63.10 $ 77.75 $ 44.92