Income Taxes - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jul. 31, 2017	Jul. 31, 2016	Jul. 31, 2017	Jul. 31, 2016
Income Tax Contingency [Line Items]
Effective tax rates	4.80%	42.30%	4.90%	40.80%
Accounting Standards Update 2016-09 [Member]
Income Tax Contingency [Line Items]
Percentage of decrease in effective tax rate due to recognition of excess tax benefits	39.00%		38.00%
Accounting Standards Update 2016-09 [Member] | Effect on Provision for Income Taxes [Member]
Income Tax Contingency [Line Items]
Excess tax benefits recognized	$ 14.8		$ 28.7