Debt (Details) (USD $) In Millions, unless otherwise specified	Jan. 31, 2013	Apr. 30, 2012
Debt Instrument [Line Items]
Long-term debt, including current portion	$ 1,250.3	$ 505.5
Current portion of long-term debt	253.7	2.7
Long-term debt	996.6	502.8
Five Point Zero Percent Notes Due In Fiscal Two Thousand Fourteen [Member]
Debt Instrument [Line Items]
Long-term debt, including current portion	251.1	251.4
Two Point Five Percent Notes Due In Fiscal Two Thousand Sixteen [Member]
Debt Instrument [Line Items]
Long-term debt, including current portion	249.1	248.8
One Point Zero Percent Notes Due in Fiscal Two Thousand Eighteen [Member]
Debt Instrument [Line Items]
Long-term debt, including current portion	249.2	0
Two Point Two Five Percent Notes Due in Fiscal Two Thousand Twenty Three [Member]
Debt Instrument [Line Items]
Long-term debt, including current portion	248.4	0
Three Point Seven Five Percent Notes Due in Fiscal Two Thousand Forty Three [Member]
Debt Instrument [Line Items]
Long-term debt, including current portion	249.9	0
Long-term Debt [Member]
Debt Instrument [Line Items]
Long-term debt, including current portion	$ 2.6	$ 5.3