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
	Apr. 30, 2018	Apr. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Issued shares (shares)	0.5	1.1
Average price of issued shares (in dollars per share)	$ 88.45	$ 38.34
Weighed average grant date fair value of awards granted under the ESPP (in dollars per share)	$ 37.64	$ 25.13