Net Income Per Share - Additional Information (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended
Apr. 30, 2015	Apr. 30, 2014
Earnings Per Share [Abstract]
Net income	$ 19.1us-gaap_NetIncomeLoss	$ 28.3us-gaap_NetIncomeLoss
Denominator:
Denominator for basic net income per share—weighted average shares (shares)	227,200,000	227,000,000
Effect of dilutive securities (shares)	4,500,000	4,600,000
Denominator for dilutive net income per share (shares)	231,700,000	231,600,000
Basic net income per share (in usd per share)	$ 0.08us-gaap_EarningsPerShareBasic	$ 0.12us-gaap_EarningsPerShareBasic
Diluted net income per share (in usd per share)	$ 0.08us-gaap_EarningsPerShareDiluted	$ 0.12us-gaap_EarningsPerShareDiluted
Potentially dilutive shares excluded from the computation of diluted net income per share (shares)	300,000	0

Securities (including those issuable pursuant to contingent stock agreements) that could potentially dilute basic earnings per share (EPS) or earnings per unit (EPU) in the future that were not included in the computation of diluted EPS or EPU because to do so would increase EPS or EPU amounts or decrease loss per share or unit amounts for the period presented.

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The amount of net income (loss) for the period per each share of common stock or unit outstanding during the reporting period.

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The amount of net income (loss) for the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period.

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The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

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The sum of dilutive potential common shares or units used in the calculation of the diluted per-share or per-unit computation.

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The average number of shares or units issued and outstanding that are used in calculating diluted EPS or earnings per unit (EPU), determined based on the timing of issuance of shares or units in the period.

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Number of [basic] shares or units, after adjustment for contingently issuable shares or units and other shares or units not deemed outstanding, determined by relating the portion of time within a reporting period that common shares or units have been outstanding to the total time in that period.

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