Employee Retirement Plans (Details 2) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2011
Pension Benefit [Member]
Components of Net periodic pension expense and other postretirement benefit costs [Abstract]
Service cost	$ 11.3	$ 9.3	$ 8.8
Interest cost	23.8	25.1	24.1
Expected return on assets	(27.8)	(26.2)	(25.8)
Curtailment gain
Amortization of:
Prior service cost (benefit)	0.3	0.2	0.2
Actuarial loss	15.1	8.4	7.5
Net benefit cost (income)	22.7	16.8	14.8
Change in associated regulatory liabilities
Net benefit cost after change in regulatory liabilities	22.7	16.8	14.8
Other Postretirement Benefits [Member]
Components of Net periodic pension expense and other postretirement benefit costs [Abstract]
Service cost	0.6	0.4	0.4
Interest cost	0.9	1.1	1.1
Expected return on assets	(0.5)	(0.5)	(0.5)
Curtailment gain			(3.2)
Amortization of:
Prior service cost (benefit)	(0.3)	(0.3)	(0.7)
Actuarial loss	0.4	0.3	0.4
Net benefit cost (income)	1.1	1.0	(2.5)
Change in associated regulatory liabilities	3.3	3.2	3.1
Net benefit cost after change in regulatory liabilities	$ 4.4	$ 4.2	$ 0.6