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Stock-based Compensation Expense (Summary of ESPP Activity) (Details) - Employee Stock - 1998 Employee Qualified Stock Purchase Plan - $ / shares
shares in Millions
3 Months Ended 9 Months Ended
Oct. 31, 2019
Oct. 31, 2018
Oct. 31, 2019
Oct. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Issued shares (shares) 0.4 0.5 0.9 1.0
Average price of issued shares (in dollars per share) $ 105.42 $ 92.50 $ 102.20 $ 90.25
Weighed average grant date fair value of awards granted under the ESPP (in dollars per share) $ 44.77 $ 47.21 $ 47.78 $ 42.75