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Equity Incentive Plan (Details) - USD ($)
$ / shares in Units, $ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]    
Options, Outstanding, Number, Beginning 1,215,481  
Options, Grants in Period, Net of Forfeitures 46,877  
Options, Exercises in Period (245,562)  
Options, Forfeitures in Period (8,320)  
Options, Outstanding, Number, Ending 1,008,476  
Options, Exercisable, Number 755,407  
Aggregate Intrinsic Value [Abstract]    
Options, Outstanding, Intrinsic Value $ 392.2 $ 274.6
Options, Exercisable, Intrinsic Value $ 331.5  
Weighted Average Exercise Price [Abstract]    
Options, Outstanding, Weighted Average Exercise Price, Beginning $ 192.63  
Options, Grants in Period, Weighted Average Exercise Price 671.60  
Options, Exercises in Period, Weighted Average Exercise Price 116.67  
Options, Forfeitures in Period, Weighted Average Exercise Price 241.83  
Options, Outstanding, Weighted Average Exercise Price, Ending 232.99  
Options, Exercisable, Weighted Average Exercise Price $ 180.66