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Retirement Plans and Other Postretirement Benefits | 6 Months Ended
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Jun. 30, 2017
Compensation and Retirement Disclosure [Abstract] |
Retirement Plans and Other Postretirement Benefits | Retirement Plans and Other Postretirement Benefits Pinnacle West sponsors a qualified defined benefit and account balance pension plan, a non-qualified supplemental excess benefit retirement plan, and an other postretirement benefit plan for the employees of Pinnacle West and our subsidiaries. Pinnacle West uses a December 31 measurement date for its pension and other postretirement benefit plans. The market-related value of our plan assets is their fair value at the measurement dates. Because of plan changes in September 2014, the Company is currently in the process of seeking IRS approval to move approximately $145 million of the other postretirement benefit trust assets into a new trust account to pay for active union employee medical costs. In December 2016, FERC approved a methodology for determining the amount of other postretirement benefit trust assets to transfer into a new trust account to pay for active union employee medical costs. While we do not expect to transfer any funds prior to 2018, as of June 30, 2017, such methodology would result in an amount of approximately $145 million being transferred to the new trust account. The following table provides details of the plans’ net periodic benefit costs and the portion of these costs charged to expense (including administrative costs and excluding amounts capitalized as overhead construction, billed to electric plant participants or charged to the regulatory asset or liability) (dollars in thousands):
| Pension Benefits |  | Other Benefits
| Three Months Ended June 30, |  | Six Months Ended June 30, |  | Three Months Ended June 30, |  | Six Months Ended June 30,
| 2017 |  | 2016 |  | 2017 |  | 2016 |  | 2017 |  | 2016 |  | 2017 |  | 2016
Service cost — benefits earned during the period | $ | 13,669 |  |  | $ | 12,630 |  |  | $ | 27,429 |  |  | $ | 26,896 |  |  | $ | 4,201 |  |  | $ | 3,560 |  |  | $ | 8,559 |  |  | $ | 7,497 |
Interest cost on benefit obligation | 32,177 |  |  | 32,878 |  |  | 64,878 |  |  | 65,823 |  |  | 7,415 |  |  | 7,519 |  |  | 14,980 |  |  | 14,860 |
Expected return on plan assets | (43,425 | ) |  | (43,161 | ) |  | (87,135 | ) |  | (86,953 | ) |  | (13,350 | ) |  | (9,125 | ) |  | (26,701 | ) |  | (18,247 | )
Amortization of: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Prior service cost (credit) | 20 |  |  | 132 |  |  | 41 |  |  | 263 |  |  | (9,461 | ) |  | (9,471 | ) |  | (18,921 | ) |  | (18,942 | )
Net actuarial loss | 11,460 |  |  | 10,627 |  |  | 23,950 |  |  | 20,358 |  |  | 1,104 |  |  | 1,349 |  |  | 2,559 |  |  | 2,295 |
Net periodic benefit cost | $ | 13,901 |  |  | $ | 13,106 |  |  | $ | 29,163 |  |  | $ | 26,387 |  |  | $ | (10,091 | ) |  | $ | (6,168 | ) |  | $ | (19,524 | ) |  | $ | (12,537 | )
Portion of cost charged to expense | $ | 6,894 |  |  | $ | 6,433 |  |  | $ | 14,461 |  |  | $ | 12,951 |  |  | $ | (5,004 | ) |  | $ | (3,027 | ) |  | $ | (9,682 | ) |  | $ | (6,153 | )
Contributions We have made voluntary contributions of $80 million to our pension plan year-to-date in 2017. The minimum required contributions for the pension plan are zero for the next three years. We expect to make voluntary contributions up to a total of $300 million during the 2017-2019 period. We expect to make contributions of less than $1 million in total for the next three years to our other postretirement benefit plans.

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