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Defined Benefit Pension and Other Postretirement Plans (Tables)
 6 Months Ended
Mar. 31, 2020
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,
 
2020
 
2019
 
2020
 
2019
Service cost
 
$
2.8

 
$
2.6

 
$
—

 
$
0.1

Interest cost
 
5.8

 
6.8

 
0.1

 
0.2

Expected return on assets
 
(9.4
)
 
(9.1
)
 
(0.2
)
 
(0.2
)
Curtailment gain
 
(1.1
)
 
—

 
—

 
—

Amortization of:
 
 
 
 
 
 
 
 
Prior service benefit
 
—

 
—

 
(0.1
)
 
(0.2
)
Actuarial loss
 
3.8

 
2.0

 
0.1

 
—

Net cost (benefit)
 
1.9

 
2.3

 
(0.1
)
 
(0.1
)
Change in associated regulatory liabilities
 
—

 
—

 
(0.4
)
 
(0.4
)
Net cost (benefit) after change in regulatory liabilities
 
$
1.9

 
$
2.3

 
$
(0.5
)
 
$
(0.5
)
 
 
 
 
 
 
 
 
 
 
 
Pension Benefits
 
Other Postretirement Benefits
Six Months Ended March 31,
 
2020
 
2019
 
2020
 
2019
Service cost
 
$
5.6

 
$
5.1

 
$
0.1

 
$
0.1

Interest cost
 
11.6

 
13.6

 
0.3

 
0.4

Expected return on assets
 
(18.8
)
 
(18.1
)
 
(0.4
)
 
(0.4
)
Curtailment gain
 
(1.1
)
 
—

 
—

 
—

Amortization of:
 
 
 
 
 
 
 
 
Prior service cost (benefit)
 
0.1

 
0.1

 
(0.2
)
 
(0.3
)
Actuarial loss
 
7.5

 
3.9

 
0.1

 
—

Net cost (benefit)
 
4.9

 
4.6

 
(0.1
)
 
(0.2
)
Change in associated regulatory liabilities
 
—

 
—

 
(0.7
)
 
(0.7
)
Net cost (benefit) after change in regulatory liabilities
 
$
4.9

 
$
4.6

 
$
(0.8
)
 
$
(0.9
)