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Stock-Based Compensation - Valuation Assumptions (Details) - Restricted Stock Awards, Market Conditions
12 Months Ended
Dec. 31, 2017
$ / shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Share Price $ 127.51
Expected Volatility 24.70%
Risk Free Interest Rate 1.90%
Expected Term 3 years
Weighted Avg. Grant Date Fair Value (USD per share) $ 119.88