Defined Benefit Pension and Other Postretirement Plans (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Mar. 31, 2012	Mar. 31, 2011
Pension Benefits [Member]
Components of net periodic pension expense and other postretirement benefit costs [Abstract]
Service cost	$ 2.1	$ 2.1	$ 4.3	$ 4.4
Interest cost	6.1	6.1	12.2	12.0
Expected return on assets	(6.4)	(6.4)	(12.8)	(12.9)
Amortization of:
Prior service cost (benefit)	0.1	0.1	0.1	0.1
Actuarial loss	2.1	1.7	4.2	4.0
Net benefit cost	4.0	3.6	8.0	7.6
Change in associated regulatory liabilities
Net expense	4.0	3.6	8.0	7.6
Other Postretirement Benefits [Member]
Components of net periodic pension expense and other postretirement benefit costs [Abstract]
Service cost	0.1	0.1	0.2	0.2
Interest cost	0.2	0.3	0.5	0.6
Expected return on assets	(0.1)	(0.1)	(0.3)	(0.3)
Amortization of:
Prior service cost (benefit)	(0.1)	(0.2)	(0.2)	(0.3)
Actuarial loss	0.1	0.1	0.2	0.2
Net benefit cost	0.2	0.2	0.4	0.4
Change in associated regulatory liabilities	0.8	0.8	1.6	1.6
Net expense	$ 1.0	$ 1.0	$ 2.0	$ 2.0