Employee Retirement Plans - Net Periodic Pension Expense and Other Postretirement Benefit Costs (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2012
Pension Benefit
Defined Benefit Plan Disclosure
Service cost	$ 9.4	$ 11.3	$ 9.3
Interest cost	26.1	23.8	25.1
Expected return on assets	(29.7)	(27.8)	(26.2)
Amortization of:
Prior service cost (benefit)	0.3	0.3	0.2
Actuarial loss	7.7	15.1	8.4
Net benefit cost	13.8	22.7	16.8
Change in associated regulatory liabilities
Net benefit cost after change in regulatory liabilities	13.8	22.7	16.8
Other Postretirement Benefits
Defined Benefit Plan Disclosure
Service cost	0.5	0.6	0.4
Interest cost	0.9	0.9	1.1
Expected return on assets	(0.6)	(0.5)	(0.5)
Amortization of:
Prior service cost (benefit)	(0.5)	(0.3)	(0.3)
Actuarial loss	0	0.4	0.3
Net benefit cost	0.3	1.1	1.0
Change in associated regulatory liabilities	3.7	3.3	3.2
Net benefit cost after change in regulatory liabilities	$ 4.0	$ 4.4	$ 4.2