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Defined Benefit Pension and Other Postretirement Plans (Tables)
 3 Months Ended
Dec. 31, 2012
Defined Benefit Pension Plans and Defined Benefit Postretirement Plans Disclosure [Abstract]  
Component of net periodic pension expense and other postretirement benefit costs
Net periodic pension expense and other postretirement benefit costs include the following components:
 
 
Pension Benefits
 
Other
Postretirement Benefits
 
 
Three Months Ended December 31,
 
Three Months Ended December 31,
 
 
2012
 
2011
 
2012
 
2011
Service cost
 
$
2.8

 
$
2.1

 
$
0.2

 
$
0.1

Interest cost
 
5.9

 
6.1

 
0.2

 
0.2

Expected return on assets
 
(6.9
)
 
(6.4
)
 
(0.1
)
 
(0.1
)
Amortization of:
 
 
 
 
 
 
 
 
Prior service cost (benefit)
 
—

 
0.1

 
(0.1
)
 
(0.1
)
Actuarial loss
 
3.8

 
2.1

 
0.1

 
0.1

Net benefit cost
 
5.6

 
4.0

 
0.3

 
0.2

Change in associated regulatory liabilities
 
—

 
—

 
0.8

 
0.8

Net expense
 
$
5.6

 
$
4.0

 
$
1.1

 
$
1.0