Share-Based Payments Stock Options (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Proceeds and Excess Tax Benefit from Share-based Compensation	$ 5.9	$ 19.2
Options and Sosars [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Outstanding at the beginning of the period (in shares)	1.5
Granted (in shares)	0.2
Exercised (in shares)	(0.2)
Forfeited (in shares)	0.0
Outstanding at the end of the period (in shares)	1.5		1.5
Share-based Compensation Arrangement by Share-based Payment Award, Options, Expected to Vest, Outstanding, Number	0.3
Exercisable (in shares)	1.2
Weighted-average exercise price of shares outstanding, beginning of the period (in dollars per share)	$ 63.60
Weighted-average exercise price of shares granted (in dollars per share)	78.79
Weighted-average exercise price of shares exercised (in dollars per share)	50.28
Weighted-average exercise price of shares forfeited (in dollars per share)	0.00
Weighted-average exercise price of shares outstanding, end of the period (in dollars per share)	67.04		$ 63.60
Share-based Compensation Arrangement by Share-based Payment Award, Options, Expected to Vest, Outstanding, Weighted Average Exercise Price	85.32
Weighted-average exercise price of shares exercisable (in dollars per share)	$ 61.35
Weighted-average remaining contractual life, outstanding (in years)	4 years 11 months 5 days		4 years 6 months 24 days
Share-based Compensation Arrangement by Share-based Payment Award, Options, Expected to Vest, Outstanding, Weighted Average Remaining Contractual Term	8 years 9 months 5 days
Weighted-average remaining contractual life, exercisable (in years)	3 years 8 months 24 days
Aggregate intrinsic value of shares outstanding (in dollars)	$ 31.3
Aggregate intrinsic value of shares outstanding (in dollars)	14.2		$ 31.3
Share-based Compensation Arrangement by Share-based Payment Award, Options, Expected to Vest, Outstanding, Aggregate Intrinsic Value	0.0
Aggregate intrinsic value of shares exercisable (in dollars)	$ 14.2