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Defined Benefit Pension and Other Postretirement Plans (Tables)
 6 Months Ended
Mar. 31, 2019
Retirement Benefits [Abstract]  
Components of Net Periodic Pension Expense and Other Postretirement Benefit Costs
Net periodic pension cost and other postretirement benefit cost include the following components:
 
 
Pension Benefits
 
Other Postretirement Benefits
Three Months Ended March 31,
 
2019
 
2018
 
2019
 
2018
Service cost
 
$
2.6

 
$
2.8

 
$
0.1

 
$
0.2

Interest cost
 
6.8

 
6.5

 
0.2

 
0.2

Expected return on assets
 
(9.1
)
 
(8.6
)
 
(0.2
)
 
(0.2
)
Amortization of:
 
 
 
 
 
 
 
 
Prior service cost (benefit)
 
—

 
0.1

 
(0.2
)
 
—

Actuarial loss
 
2.0

 
3.3

 
—

 
—

Net benefit cost (benefit)
 
2.3

 
4.1

 
(0.1
)
 
0.2

Change in associated regulatory liabilities
 
—

 
—

 
(0.4
)
 
(0.1
)
Net benefit cost (benefit) after change in regulatory liabilities
 
$
2.3

 
$
4.1

 
$
(0.5
)
 
$
0.1

 
 
 
 
 
 
 
 
 
Pension Benefits
 
Other Postretirement Benefits
Six Months Ended March 31,
 
2019
 
2018
 
2019
 
2018
Service cost
 
$
5.1

 
$
5.6

 
$
0.1

 
$
0.4

Interest cost
 
13.6

 
13.0

 
0.4

 
0.4

Expected return on assets
 
(18.1
)
 
(17.2
)
 
(0.4
)
 
(0.4
)
Amortization of:
 
 
 
 
 
 
 
 
Prior service cost (benefit)
 
0.1

 
0.2

 
(0.3
)
 
(0.1
)
Actuarial loss
 
3.9

 
6.6

 
—

 
—

Net benefit cost (benefit)
 
4.6

 
8.2

 
(0.2
)
 
0.3

Change in associated regulatory liabilities
 
—

 
—

 
(0.7
)
 
(0.2
)
Net benefit cost (benefit) after change in regulatory liabilities
 
$
4.6

 
$
8.2

 
$
(0.9
)
 
$
0.1