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Quarterly Financial Information (Parenthetical) (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		12 Months Ended
	Jan. 31, 2018	Jul. 31, 2016	Apr. 30, 2018
Quarterly Financial Information [Line Items]
Income tax benefit by effect of tax reform	$ 9.4		$ 8.7
ASU 2016-09
Quarterly Financial Information [Line Items]
Net income increased		$ 2.6
Net income per basic share increased		$ 0.05
Net income per diluted share increased		$ 0.05

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