Defined Benefit Pension and Other Postretirement Plans - Components of Net Periodic Pension Expense and Other Postretirement Benefit Costs (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Pension Benefits
Defined Benefit Plan Disclosure
Service cost	$ 2.4	$ 2.3
Interest cost	6.3	6.4
Expected return on assets	(7.9)	(7.3)
Amortization of:
Prior service cost (benefit)	0.1	0.1
Actuarial loss	2.5	1.9
Net benefit cost	3.4	3.4
Change in associated regulatory liabilities	0	0
Net expense	3.4	3.4
Other Postretirement Benefits
Defined Benefit Plan Disclosure
Service cost	0.2	0.1
Interest cost	0.2	0.2
Expected return on assets	(0.2)	(0.1)
Amortization of:
Prior service cost (benefit)	(0.1)	(0.1)
Actuarial loss	0	0
Net benefit cost	0.1	0.1
Change in associated regulatory liabilities	0.9	0.9
Net expense	$ 1.0	$ 1.0