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Interim Pension and Other Postretirement Benefits (Table)
 3 Months Ended
Dec. 31, 2017
Retirement Benefits, Description [Abstract]  
Components of net periodic pension cost table
The components of our net periodic pension cost for our pension and other postretirement benefit plans for the three months ended December 31, 2017 and 2016 are presented in the following table. Most of these costs are recoverable through our tariff rates; however, a portion of these costs is capitalized into our rate base. The remaining costs are recorded as a component of operation and maintenance expense.
 
Three Months Ended December 31
 
Pension Benefits
 
Other Benefits
 
2017
 
2016
 
2017
 
2016
 
(In thousands)
Components of net periodic pension cost:
 
 
 
 
 
 
 
Service cost
$
4,560

 
$
5,216

 
$
3,020

 
$
3,109

Interest cost
6,430

 
6,297

 
2,727

 
2,670

Expected return on assets
(6,917
)
 
(6,994
)
 
(2,002
)
 
(1,796
)
Amortization of prior service cost (credit)
(58
)
 
(58
)
 
3

 
(411
)
Amortization of actuarial (gain) loss
3,089

 
4,249

 
(1,618
)
 
(707
)
Net periodic pension cost
$
7,104

 
$
8,710

 
$
2,130

 
$
2,865

 
 
 
 
 
 
 
 
Schedule of assumptions used table text block
The assumptions used to develop our net periodic pension cost for the three months ended December 31, 2017 and 2016 are as follows:
 
 
Pension Benefits
 
Other Benefits
 
 
2017
 
2016
 
2017
 
2016
Discount rate
 
3.89%
 
3.73%
 
3.89%
 
3.73%
Rate of compensation increase
 
3.50%
 
3.50%
 
N/A
 
N/A
Expected return on plan assets
 
6.75%
 
7.00%
 
4.29%
 
4.45%