Defined Benefit Pension and Other Postretirement Plans (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Dec. 31, 2013	Dec. 31, 2012
Pension Benefits [Member]
Components of net periodic pension expense and other postretirement benefit costs [Abstract]
Service cost	$ 2.3	$ 2.8
Interest cost	6.4	5.9
Expected return on assets	(7.3)	(6.9)
Amortization of:
Prior service cost (benefit)	0
Actuarial loss	2.0	3.8
Net benefit cost	3.4	5.6
Change in associated regulatory liabilities	0	0
Net expense	3.4	5.6
Other Postretirement Benefits [Member]
Components of net periodic pension expense and other postretirement benefit costs [Abstract]
Service cost	0.1	0.2
Interest cost	0.2	0.2
Expected return on assets	(0.1)	(0.1)
Amortization of:
Prior service cost (benefit)	(0.1)	(0.1)
Actuarial loss		0.1
Net benefit cost	0.1	0.3
Change in associated regulatory liabilities	0.9	0.8
Net expense	$ 1.0	$ 1.1