Employee Retirement Plans (Details 2) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2010
Pension Benefit [Member]
Components of Net periodic pension expense and other postretirement benefit costs [Abstract]
Service cost	$ 9.3	$ 8.8	$ 8.7
Interest cost	25.1	24.1	23.5
Expected return on assets	(26.2)	(25.8)	(25.8)
Curtailment gain
Settlement loss			1.0
Amortization of:
Prior service cost (benefit)	0.2	0.2
Actuarial loss	8.4	7.5	5.9
Net benefit cost (income)	16.8	14.8	13.3
Change in associated regulatory liabilities
Net benefit cost after change in regulatory liabilities	16.8	14.8	13.3
Other Postretirement Benefits [Member]
Components of Net periodic pension expense and other postretirement benefit costs [Abstract]
Service cost	0.4	0.4	0.4
Interest cost	1.1	1.1	1.1
Expected return on assets	(0.5)	(0.5)	(0.5)
Curtailment gain		(3.2)
Settlement loss
Amortization of:
Prior service cost (benefit)	(0.3)	(0.7)	(0.4)
Actuarial loss	0.3	0.4	0.1
Net benefit cost (income)	1.0	(2.5)	0.7
Change in associated regulatory liabilities	3.2	3.1	3.1
Net benefit cost after change in regulatory liabilities	$ 4.2	$ 0.6	$ 3.8