Stock-Based Compensation (Narrative) (Details) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended
May 31, 2013	May 31, 2012	May 31, 2011
Stock Compensation Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized under plan (shares)	14,000,000
Shares reserved for future issuance (shares)	6,148,994
Stock-based compensation	$ 23.3	$ 20.3	$ 15.2
The total income tax benefit recognized in the consolidated income statement for share-based compensation arrangements	6.6	5.6	4.5
Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum term after grant options may be exercised (years)	10 years
Options vesting period (years)	10 years
Weighted-average fair value of stock options granted (dollars per share)	$ 9.55	$ 9.48	$ 8.04
Intrinsic value of options exercised during the period	3.7	0.6
Proceeds from stock options exercised	14.8	3.3
Fair value of vested stock options	13.2	12.9	9.0
Aggregate intrinsic value of outstanding options	63.8
Aggregate intrinsic value of exercisable options	12.4
The weighted-average remaining contractual term of stock options exercisable (years)	2 years 3 months 12 days
Stock Options [Member] \| Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options vesting period (years)	3 years
Stock Options [Member] \| Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options vesting period (years)	10 years
Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period of awards (years)	3 years
Unrecognized compensation cost related to unvested stock options and restricted stock	$ 62.3
Weighted average over which stock options and restricted stock costs will be recognized (years)	2 years 6 months 11 days

Share-based Compensation Arrangement By Share Based Payment Award Award Exercises After Grant Term Maximum

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- Definition

Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Award Vesting Period

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Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees.

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Aggregate number of common shares reserved for future issuance.

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As of the balance sheet date, the aggregate unrecognized cost of equity-based awards made to employees under equity-based compensation awards that have yet to vest.

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Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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The total recognized tax benefit related to compensation cost for equity-based payment arrangements recognized in income during the period.

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The cash inflow associated with the amount received from holders exercising their stock options. This item inherently excludes any excess tax benefit, which the entity may have realized and reported separately.

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Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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The total fair value of equity-based awards for which the grantee gained the right during the reporting period, by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash in accordance with the terms of the arrangement.

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The maximum number of shares (or other type of equity) originally approved (usually by shareholders and board of directors), net of any subsequent amendments and adjustments, for awards under the equity-based compensation plan. As stock or unit options and equity instruments other than options are awarded to participants, the shares or units remain authorized and become reserved for issuance under outstanding awards (not necessarily vested).

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Amount of difference between fair value of the underlying shares reserved for issuance and exercise price of vested portions of options outstanding and currently exercisable.

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Weighted average remaining contractual term for vested portions of options outstanding and currently exercisable or convertible, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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The total accumulated difference between fair values of underlying shares on dates of exercise and exercise price on options which were exercised (or share units converted) into shares during the reporting period under the plan.

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The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology.

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Amount of difference between fair value of the underlying shares reserved for issuance and exercise price of options outstanding.

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