Stock-Based Compensation (Narrative) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
Apr. 30, 2015	Apr. 30, 2014	Apr. 30, 2013
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]
Stock-based compensation expenses	$ 26.1	$ 20.1	$ 15.3
Tax benefit of stock-based compensation	9.9	7.6	5.8
Realized tax benefits	12.5	10.6	5.0
Shares reserved for future awards under stock-based compensation plans	9.6
2003 Long-Term Executive Compensation Plan
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]
Vesting period (in years), minimum	3 years
Nonvested Shares
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]
Fair value of shares vesting during period	14.3	8.6	7.2
Unrecognized compensation cost	31.2
Weighted-average period of recognition (years)	2 years
Stock Options
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]
Intrinsic value of options exercised	8.4	13.6	6.0
Unrecognized compensation cost	0.2
Weighted-average period of recognition (years)	1 year
Nonvested Shares And Performance Nonvested
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]
Fair value of shares vesting during period		$ 0	$ 0
Minimum \| 2003 Long-Term Executive Compensation Plan
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]
Vesting period (in years), minimum	3 years

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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-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

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Unrecognized cost of unvested share-based compensation awards.

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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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Disclosure of the aggregate tax benefit realized from the exercise of stock options and the conversion of similar instruments during the annual period.

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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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Fair value of share-based awards for which the grantee gained the right by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash.

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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 accumulated difference between fair value of underlying shares on dates of exercise and exercise price on options exercised (or share units converted) into shares.

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