Table:
|  | 2019 | 2018 | 2017 |
| --- | --- | --- | --- |
| Qualified Plan: |  |  |  |
| Interest cost | $19,825 | $19,463 | $19,889 |
| Expected return on plan assets | (26,334) | (26,467) | (26,811) |
| Actuarial loss | 2,754 | 3,331 | 4,455 |
| Net periodic benefit credit | $(3,755) | $(3,673) | $(2,467) |
| SERP: |  |  |  |
| Service cost | $— | $490 | $855 |
| Interest cost | 3,080 | 2,894 | 2,850 |
| Actuarial loss | 1,207 | 1,538 | 1,659 |
| Amortization of unrecognized prior service cost | 115 | 146 | 153 |
| Net periodic benefit cost | $4,402 | $5,068 | $5,517 |
| Postretirement health plans: |  |  |  |
| Interest cost | $997 | $955 | $1,003 |
| Actuarial (gain) loss | (159) | (27) | 162 |
| Net periodic benefit cost | $838 | $928 | $1,165 |
Paragraphs:
Net periodic benefit cost — The components of the fiscal year net periodic benefit cost were as follows (in thousands):
Changes in presentation —As discussed in Note 1, Nature of Operations and Summary of Significant Accounting Policies, we adopted ASU 2017-07 during the first quarter of 2019 using the retrospective method, which changed the financial statement presentation of service costs and the other components of net periodic benefit cost. The service cost component continues to be included in operating income; however, the other components are now presented in a separate line below earnings from operations captioned “Other pension and post-retirement expenses, net” in our consolidated statements of earnings. Further, in connection with the adoption, plan administrative expenses historically presented as a component of service cost are now presented as a component of expected return on plan assets. The prior year components of net periodic benefit costs and assumptions on the long-term rate of return on assets have been recast to conform to current year presentation.
Prior service costs are amortized on a straight-line basis from date of participation to full eligibility. Unrecognized gains or losses are amortized using the “corridor approach” under which the net gain or loss in excess of 10% of the greater of the PBO or the market-related value of the assets, if applicable, is amortized. For our Qualified Plan, actuarial losses are amortized over the average future expected lifetime of all participants expected to receive benefits. For our SERP, actuarial losses are amortized over the expected remaining future lifetime for inactive participants, and for our postretirement health plans, actuarial losses are amortized over the expected remaining future lifetime of inactive participants expected to receive benefits.
Question:
How are prior service costs amortized?
Answer: On a straight-line basis from date of participation to full eligibility