Pension and Other Postretirement Benefits (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Oct. 31, 2011	Oct. 31, 2010	Oct. 31, 2011	Oct. 31, 2010
Pension Benefits [Member]
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Service cost	$ 4.0	$ 3.9	$ 8.0	$ 7.8
Interest cost	8.5	8.3	17.0	16.7
Expected return on plan assets	(10.0)	(9.1)	(20.1)	(18.1)
Amortization of:
Prior service cost	0.2	0.2	0.4	0.4
Net actuarial loss	4.8	4.7	9.7	9.3
Net expense	7.5	8.0	15.0	16.1
Other Postretirement Benefits [Member]
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Service cost	0.4	0.3	0.7	0.6
Interest cost	0.7	0.8	1.5	1.6
Amortization of:
Prior service cost	0.2		0.3
Net actuarial loss				0.1
Net expense	$ 1.3	$ 1.1	$ 2.5	$ 2.3