Deferred Compensation (Additional Information) (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
	Jul. 31, 2018	Jul. 31, 2018	Jan. 31, 2018
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]
Marketable securities	$ 402.5	$ 402.5
Costs to obtain a contract	82.5	82.5
Amortization of costs to obtain a contract	26.9	53.1
Rabbi Trust Member
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]
Deferred compensation liability	62.0	62.0	$ 59.0
Deferred compensation liability, current	3.0	3.0	3.4
Deferred compensation liability, non-current	59.0	59.0	55.6
Mutual funds
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]
Trading securities	$ 62.0	$ 62.0	$ 59.0