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Defined Benefit Pension and Other Postretirement Plans (Tables)
 3 Months Ended
Dec. 31, 2016
Defined Benefit Pension Plans and Defined Benefit Postretirement Plans Disclosure [Abstract]  
Components 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,
 
2016
 
2015
 
2016
 
2015
Service cost
 
$
3.0

 
$
2.5

 
$
0.2

 
$
0.2

Interest cost
 
6.2

 
6.6

 
0.2

 
0.2

Expected return on assets
 
(8.3
)
 
(8.0
)
 
(0.2
)
 
(0.2
)
Amortization of:
 
 
 
 
 
 
 
 
Prior service cost (benefit)
 
0.1

 
0.1

 
(0.1
)
 
(0.1
)
Actuarial loss
 
4.1

 
2.7

 
0.1

 
—

Net benefit cost
 
5.1

 
3.9

 
0.2

 
0.1

Change in associated regulatory liabilities
 
—

 
—

 
(0.1
)
 
0.9

Net expense
 
$
5.1

 
$
3.9

 
$
0.1

 
$
1.0