Stock-based Compensation Expense (Options Activity) (Details) $ / shares in Units, shares in Millions, $ in Millions	6 Months Ended
	Jul. 31, 2019 USD ($) $ / shares shares
Number of Shares
Options outstanding beginning (in shares) | shares	0.8
Exercised (in shares) | shares	(0.2)
Options outstanding ending (in shares) | shares	0.6
Options vested and exercisable (in shares) | shares	0.2
Weighted average exercise price per share
Options outstanding beginning (in dollars per share) | $ / shares	$ 23.95
Exercised (in dollars per share) | $ / shares	22.76
Options outstanding ending (in dollars per share) | $ / shares	24.46
Vested and exercisable (in dollars per share) | $ / shares	$ 32.71
Options outstanding, weighted average remaining contractual term (in years)	6 years 9 months 18 days
Options outstanding, aggregate intrinsic value | $	$ 78.6
Options vested and exercisable, weighted average remaining contractual term (in years)	4 years
Options vested and exercisable, aggregate intrinsic value | $	$ 21.8
Stock price (in usd per share) | $ / shares	$ 156.17
Stock Options
Number of Shares
Shares available for grant (in shares) | shares	17.4