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Defined Benefit Pension and Other Postretirement Plans (Tables)
 3 Months Ended
Dec. 31, 2017
Defined Benefit Plan [Abstract]  
Schedule of 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
 
$
1,881

 
$
2,023

 
$
67

 
$
61

Interest cost
 
5,767

 
5,539

 
112

 
108

Expected return on assets
 
(7,777
)
 
(7,497
)
 
(177
)
 
(164
)
Amortization of:
 
 
 
 
 
 
 
 
Prior service cost (benefit)
 
63

 
81

 
(110
)
 
(160
)
Actuarial loss
 
2,984

 
3,707

 
24

 
28

Net benefit cost (benefit)
 
2,918

 
3,853

 
(84
)
 
(127
)
Change in associated regulatory liabilities
 
—

 
—

 
(123
)
 
(122
)
Net benefit cost (benefit) after change in regulatory liabilities
 
$
2,918

 
$
3,853

 
$
(207
)
 
$
(249
)