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, 2014	Mar. 31, 2013	Mar. 31, 2014	Mar. 31, 2013
Pension Benefits
Components of net periodic pension expense and other postretirement benefit costs
Service cost	$ 2.3	$ 2.8	$ 4.7	$ 5.7
Interest cost	6.5	5.9	12.9	11.8
Expected return on assets	(7.3)	(6.9)	(14.7)	(13.8)
Amortization of:
Prior service cost (benefit)	0.1	0.1	0.2	0.2
Actuarial loss	1.9	3.7	3.8	7.4
Net benefit cost	3.5	5.6	6.9	11.3
Change in associated regulatory liabilities	0	0	0	0
Net expense	3.5	5.6	6.9	11.3
Other Postretirement Benefits
Components of net periodic pension expense and other postretirement benefit costs
Service cost	0.1	0.2	0.3	0.3
Interest cost	0.2	0.2	0.5	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)	(0.1)
Actuarial loss	0	0.1	0	0.2
Net benefit cost	0.1	0.3	0.3	0.6
Change in associated regulatory liabilities	0.9	0.8	1.8	1.6
Net expense	$ 1.0	$ 1.1	$ 2.1	$ 2.2