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Interim Pension and Other Postretirement Benefit Plan Information
 3 Months Ended
Dec. 31, 2017
Retirement Benefits, Description [Abstract]  
Interim Pension and Postretirement Benefit Plans
Interim Pension and Other Postretirement Benefit Plan Information
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

 
 
 
 
 
 
 
 

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%

The discount rate used to compute the present value of a plan’s liabilities generally is based on rates of high-grade corporate bonds with maturities similar to the average period over which the benefits will be paid. Generally, our funding policy has been to contribute annually an amount in accordance with the requirements of the Employee Retirement Income Security Act of 1974. In accordance with the Pension Protection Act of 2006 (PPA), we determined the funded status of our plan as of January 1, 2017. Based on that determination, we were not required to make a minimum contribution to our defined benefit plan during the first quarter of fiscal 2018.
We contributed $3.9 million to our other post-retirement benefit plans during the three months ended December 31, 2017. We expect to contribute a total of between $10 million and $20 million to these plans during fiscal 2018.