Pension and Other Postretirement Benefits (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Jul. 31, 2012	Jul. 31, 2011
Pension Benefits [Member]
Other Postretirement Benefits:
Service cost	$ 4.9	$ 4.0
Interest cost	8.8	8.5
Expected return on plan assets	(10.2)	(10.1)
Amortization of:
Prior service cost	0.2	0.2
Net actuarial loss	7.0	4.9
Net expense	10.7	7.5
Other Postretirement Benefits [Member]
Other Postretirement Benefits:
Service cost	0.4	0.4
Interest cost	0.7	0.8
Amortization of:
Prior service cost	0.2	0.1
Net expense	$ 1.3	$ 1.3