Deferred Compensation (Additional Information) (Details) - USD ($) $ in Millions	3 Months Ended
	Apr. 30, 2018	Jan. 31, 2018
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]
Marketable securities	$ 371.4
Costs to obtain a contract	93.3
Amortization of costs to obtain a contract	26.2
Rabbi Trust Member
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]
Deferred compensation liability	59.4	$ 59.0
Deferred compensation liability, current	2.9	3.4
Deferred compensation liability, non-current	56.5	55.6
Mutual funds
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]
Trading securities	$ 59.4	$ 59.0