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Defined Benefit Pension and Other Postretirement Plans (Tables)
 9 Months Ended
Jun. 30, 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 June 30,
 
2019
 
2018
 
2019
 
2018
Service cost
 
$
2.5

 
$
2.8

 
$
—

 
$
0.2

Interest cost
 
6.8

 
6.5

 
0.2

 
0.3

Expected return on assets
 
(9.0
)
 
(8.7
)
 
(0.2
)
 
(0.2
)
Amortization of:
 
 
 
 
 
 
 
 
Prior service cost (benefit)
 
0.1

 
—

 
(0.1
)
 
—

Actuarial loss (gain)
 
1.9

 
3.4

 
—

 
—

Net benefit cost (benefit)
 
2.3

 
4.0

 
(0.1
)
 
0.3

Change in associated regulatory liabilities
 
—

 
—

 
(0.3
)
 
(0.1
)
Net benefit cost (benefit) after change in regulatory liabilities
 
$
2.3

 
$
4.0

 
$
(0.4
)
 
$
0.2

 
 
 
 
 
 
 
 
 
 
 
Pension Benefits
 
Other Postretirement Benefits
Nine Months Ended June 30,
 
2019
 
2018
 
2019
 
2018
Service cost
 
$
7.6

 
$
8.4

 
$
0.1

 
$
0.6

Interest cost
 
20.4

 
19.5

 
0.5

 
0.7

Expected return on assets
 
(27.1
)
 
(25.9
)
 
(0.6
)
 
(0.6
)
Amortization of:
 
 
 
 
 
 
 
 
Prior service cost (benefit)
 
0.2

 
0.2

 
(0.3
)
 
(0.2
)
Actuarial loss (gain)
 
5.8

 
10.0

 
(0.1
)
 
—

Net benefit cost (benefit)
 
6.9

 
12.2

 
(0.4
)
 
0.5

Change in associated regulatory liabilities
 
—

 
—

 
(1.0
)
 
(0.4
)
Net benefit cost (benefit) after change in regulatory liabilities
 
$
6.9

 
$
12.2

 
$
(1.4
)
 
$
0.1