Stock-Based Compensation (Narrative) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Apr. 30, 2013	Apr. 30, 2012	Apr. 30, 2011
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]
Share-based compensation expense, net of tax benefits	$ 15.3	$ 15.0	$ 14.5
Tax benefit of stock-based compensation	5.8	5.4	5.4
Stock-based compensation expenses	15.3	14.2	10.5
Excess tax benefits from stock-based compensation, financing activities	0.4	0.1	0.5
Realized tax benefits	5.0	4.4	4.4
Shares reserved for future awards under stock-based compensation plans	11.3
Stock Options
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]
Intrinsic value of options exercised	6.0	1.0	1.8
Unrecognized compensation cost	3.6
Weighted-average period of recognition (years)	1 year
Nonvested Shares
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]
Fair value of shares vesting during period	6.9	11.9	13.0
Unrecognized compensation cost	14.7
Weighted-average period of recognition (years)	2 years
Nonvested Shares And Performance-Based Nonvested
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]
Fair value of shares vesting during period	0.3	0	0
Unrecognized compensation cost	$ 7.0
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
Retirement age	60 years
Early retirement age	55 years
Service period (in years)	5 years
Contractual term	1 year
Performance period of total shareholder return (years)	3 years
2008 Deferred Stock Unit Plan [Member]
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]
Criteria for Settlement of Shares, Months after Directors Seperation	6 months
Share Based Compensation Arrangement by Share-based Payment Award, Criteria for Settlement of Shares, Days after Directors Death	90 days
2000 Employee Stock Purchase Plan
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]
Purchase price discount	90.00%
Share-based Compensation Arrangement by Share-based Payment Award, Discount from Market Price, Purchase Date	15.00%
Discount from market price	85.00%
Minimum [Member] | 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
Maximum [Member] | 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	4 years