Stock-based Compensation Expense (Summary of ESPP Activity) (Details) - Employee Stock - Employee Qualified Stock Purchase Plan 1998 ESP Plan - $ / shares shares in Millions	3 Months Ended		9 Months Ended
	Oct. 31, 2018	Oct. 31, 2017	Oct. 31, 2018	Oct. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Issued shares (shares)	0.5	0.9	1.0	2.0
Average price of issued shares (in dollars per share)	$ 92.50	$ 39.92	$ 90.25	$ 39.03
Weighed average grant date fair value of awards granted under the ESPP (in dollars per share)	$ 47.21	$ 33.04	$ 42.75	$ 32.41