Net Income Per Share - Additional Information (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jul. 31, 2014	Jul. 31, 2013	Jul. 31, 2014	Jul. 31, 2013
Earnings Per Share [Abstract]
Net income	$ 31.3	$ 61.7	$ 59.6	$ 117.3
Denominator for basic net income per share-weighted average shares (in shares)	227,300,000	223,100,000	227,100,000	223,600,000
Effect of dilutive securities (in shares)	5,100,000	5,200,000	5,300,000	5,700,000
Denominator for dilutive net income per share (in shares)	232,400,000	228,300,000	232,400,000	229,300,000
Basic net income per share (in usd per share)	$ 0.14	$ 0.28	$ 0.26	$ 0.52
Diluted net income per share (in usd per share)	$ 0.13	$ 0.27	$ 0.26	$ 0.51
Potentially dilutive shares excluded from the computation of diluted net income per share	0	7,200,000	0	7,600,000