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

text: Net periodic benefit cost — The components of the fiscal year net periodic benefit cost were as follows (in thousands):
text: 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.
text: 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.

Questions:
question: How are prior service costs amortized?
answer: On a straight-line basis from date of participation to full eligibility
question: For SERP, how are actuarial losses amortized?
answer: Over the expected remaining future lifetime for inactive participants
question: For SERP, what is the net periodic benefit cost for 2019?
answer: $4,402
scale: thousand
question: For Qualified Plan, what is the difference in interest cost between 2018 and 2019?
answer: 362
derivation: $19,825-$19,463
scale: thousand
question: For SERP, what is the average actuarial loss for the years 2017-2019?
answer: 1468
derivation: (1,207+1,538+1,659)/3
scale: thousand
question: For Postretirement health plans, what is the percentage constitution of interest cost among the net periodic benefit cost in 2017?
answer: 86.09
derivation: 1,003/1,165
scale: percent