Deferred Compensation (Additional Information) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jul. 31, 2019	Jul. 31, 2018	Jul. 31, 2019	Jul. 31, 2018	Jan. 31, 2019
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]
Marketable securities	$ 67.4		$ 67.4
Costs to obtain a contract	78.6		78.6		$ 93.0
Amortization of costs to obtain a contract	25.5	$ 26.9	50.2	$ 53.1
Rabbi Trust
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]
Deferred compensation liability	67.4		67.4		60.3
Deferred compensation liability, current	6.2		6.2		5.0
Deferred compensation liability, non-current	$ 61.2		$ 61.2		$ 55.3