Pension and Other Postretirement Benefits (Details) (USD $) In Millions	3 Months Ended
	Jul. 31, 2011	Jul. 31, 2010
Pension Benefits [Member]
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Service cost	$ 4.0	$ 3.9
Interest cost	8.5	8.3
Expected return on plan assets	(10.1)	(9.1)
Amortization of:
Prior service cost	0.2	0.2
Net actuarial loss	4.9	4.7
Net expense	7.5	8.0
Other Pension Benefits [Member]
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Service cost	0.4	0.3
Interest cost	0.8	0.8
Amortization of:
Prior service cost	0.1
Net expense	$ 1.3	$ 1.1