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Defined Benefit Pension and Other Postretirement Plans (Tables)
 3 Months Ended
Dec. 31, 2017
Retirement Benefits [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,
 
2017
 
2016
 
2017
 
2016
Service cost
 
$
2.8

 
$
3.0

 
$
0.2

 
$
0.2

Interest cost
 
6.5

 
6.2

 
0.2

 
0.2

Expected return on assets
 
(8.6
)
 
(8.3
)
 
(0.2
)
 
(0.2
)
Amortization of:
 
 
 
 
 
 
 
 
Prior service cost (benefit)
 
0.1

 
0.1

 
(0.1
)
 
(0.1
)
Actuarial loss
 
3.3

 
4.1

 
0.1

 
0.1

Net benefit cost
 
4.1

 
5.1

 
0.2

 
0.2

Change in associated regulatory liabilities
 
—

 
—

 
(0.1
)
 
(0.1
)
Net benefit cost after change in regulatory liabilities
 
$
4.1

 
$
5.1

 
$
0.1

 
$
0.1