Pension and Other Postretirement Benefits (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Jan. 31, 2012	Jan. 31, 2011	Jan. 31, 2012	Jan. 31, 2011
Pension Benefits [Member]
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Service cost	$ 4.0	$ 3.9	$ 12.0	$ 11.7
Interest cost	8.5	8.3	25.5	25.0
Expected return on plan assets	(10.0)	(9.1)	(30.3)	(27.2)
Amortization of:
Prior service cost	0.2	0.2	0.7	0.7
Net actuarial loss	4.8	4.7	14.5	14.0
Net expense	7.5	8.0	22.4	24.2
Other Postretirement Benefits [Member]
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Service cost	0.4	0.3	1.1	1.0
Interest cost	0.7	0.8	2.3	2.4
Amortization of:
Prior service cost	0.2		0.4
Net actuarial loss				0.1
Net expense	$ 1.3	$ 1.1	$ 3.8	$ 3.5