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Share Based Compensation - Aggregate Intrinsic Value (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Stock Options and Stock Appreciation Rights, Exercised, Aggregate Intrinsic Value $ 13.1    
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Remaining Contractual Term 3 years 11 months    
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Weighted Average Remaining Contractual Term 2 years 8 months    
Stock Options and Stock Appreciation Rights, Outstanding, Aggregate Intrinsic Value $ 20.9    
Stock Option SAR Awards [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Stock Options and Stock Appreciation Rights, Exercised, Aggregate Intrinsic Value $ 4.4 $ 6.2 $ 6.2