Employee Retirement Plans (Details 2) (USD $) In Millions	12 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2009
Pension Benefit [Member]
Components of Net periodic pension expense and other postretirement benefit costs [Abstract]
Service cost	$ 8.8	$ 8.7	$ 7.1
Interest cost	24.1	23.5	23.3
Expected return on assets	(25.8)	(25.8)	(25.7)
Settlement loss		1.0	1.8
Amortization of:
Prior service benefit	0.2
Actuarial loss (gain)	7.5	5.9	3.8
Net benefit cost (income)	14.8	13.3	10.3
Net benefit cost after change in regulatory liabilities	14.8	13.3	10.3
Other Postretirement Benefits [Member]
Components of Net periodic pension expense and other postretirement benefit costs [Abstract]
Service cost	0.4	0.4	0.3
Interest cost	1.1	1.1	1.2
Expected return on assets	(0.5)	(0.5)	(0.6)
Curtailment gain	(3.2)
Amortization of:
Transition obligation			0.2
Prior service benefit	(0.7)	(0.4)	(0.4)
Actuarial loss (gain)	0.4	0.1	(0.1)
Net benefit cost (income)	(2.5)	0.7	0.6
Change in associated regulatory liabilities	3.1	3.1	3.3
Net benefit cost after change in regulatory liabilities	$ 0.6	$ 3.8	$ 3.9