Pension and Other Postretirement Benefits (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Jan. 31, 2013	Jan. 31, 2012	Jan. 31, 2013	Jan. 31, 2012
Pension Benefits [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	$ 4.9	$ 4.0	$ 14.7	$ 12.0
Interest cost	8.8	8.5	26.3	25.5
Expected return on plan assets	(10.2)	(10.0)	(30.5)	(30.3)
Amortization of:
Prior service cost	0.2	0.2	0.7	0.7
Net actuarial loss	7.0	4.8	21.0	14.5
Net cost	10.7	7.5	32.2	22.4
Other Postretirement Benefits [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	0.4	0.4	1.3	1.1
Interest cost	0.7	0.7	2.2	2.3
Amortization of:
Prior service cost	0.2	0.2	0.5	0.4
Net cost	$ 1.3	$ 1.3	$ 4.0	$ 3.8