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		6 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Mar. 31, 2015	Mar. 31, 2014
Pension Benefits
Defined Benefit Plan Disclosure
Service cost	$ 2.5	$ 2.3	$ 4.9	$ 4.7
Interest cost	6.3	6.5	12.6	12.9
Expected return on assets	(8.0)	(7.3)	(15.9)	(14.7)
Amortization of:
Prior service cost (benefit)	0.1	0.1	0.2	0.2
Actuarial loss	2.5	1.9	5.0	3.8
Net benefit cost	3.4	3.5	6.8	6.9
Change in associated regulatory liabilities	0	0	0	0
Net expense	3.4	3.5	6.8	6.9
Other Postretirement Benefits
Defined Benefit Plan Disclosure
Service cost	0.1	0.1	0.3	0.3
Interest cost	0.2	0.2	0.4	0.5
Expected return on assets	(0.1)	(0.1)	(0.3)	(0.3)
Amortization of:
Prior service cost (benefit)	(0.1)	(0.1)	(0.2)	(0.2)
Actuarial loss	0	0	0	0
Net benefit cost	0.1	0.1	0.2	0.3
Change in associated regulatory liabilities	1.0	0.9	1.9	1.8
Net expense	$ 1.1	$ 1.0	$ 2.1	$ 2.1