Stock-Based Compensation (Narrative) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Apr. 30, 2012 Y	Apr. 30, 2011	Apr. 30, 2010
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]
Share-based compensation expense, net of tax benefits	$ 15.0	$ 14.5	$ 29.4
Tax benefit of stock-based compensation	5.4	5.4	10.5
Stock-based compensation expenses	14.2	10.5	22.7
Excess tax benefits from stock-based compensation, financing activities	0.1	0.5	1.6
Realized tax benefits	4.4	4.4	6.6
Shares reserved for future awards under stock-based compensation plans	10.2
Performance period of total shareholder return (years)	3
Stock Options [Member]
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]
Intrinsic value of options exercised	1.0	1.8	5.4
Unrecognized compensation cost	7.1
Weighted-average period of recognition (years)	2
Nonvested Shares [Member]
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]
Fair value of shares vesting during period	9.0	11.3	12.4
Unrecognized compensation cost	13.1
Nonvested Shares And Performance-Based Nonvested [Member]
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]
Fair value of shares vesting during period	2.9	1.7	3.1
Unrecognized compensation cost	$ 1.9
Weighted-average period of recognition (years)	2
2003 Long-Term Executive Compensation Plan [Member]
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]
Vesting period (in years), minimum	three
Vesting period (in years), maximum	four
Retirement age	65
Early retirement age	55
Service period (in years)	ten
Contractual term	10
1999 Stock Option Plan [Member]
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]
Awards vesting period	two
Contractual term	29
1989 Stock Option Plan [Member]
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]
Period before settlement of shares (in months)	6
2000 Employee Stock Purchase Plan [Member]
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]
Purchase price discount	90.00%
Discount from market price	10.00%
Awards vesting period	six
Performance Shares [Member]
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]
Awards vesting period	three