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Defined Benefit Pension and Other Postretirement Plans (Details) (USD $)
In Millions, unless otherwise specified
 3 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Pension Benefits [Member]
    
Components of net periodic pension expense and other postretirement benefit costs [Abstract]    
Service cost $ 2.8 $ 2.1
Interest cost 5.9 6.1
Expected return on assets (6.9) (6.4)
Amortization of:    
Prior service cost (benefit) 0 0.1
Actuarial loss 3.8 2.1
Net benefit cost 5.6 4.0
Change in associated regulatory liabilities 0 0
Net expense 5.6 4.0
Other Postretirement Benefits [Member]
    
Components of net periodic pension expense and other postretirement benefit costs [Abstract]    
Service cost 0.2 0.1
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 0.1
Net benefit cost 0.3 0.2
Change in associated regulatory liabilities 0.8 0.8
Net expense $ 1.1 $ 1.0