Table:
|  |  | Combined Pension Plan |  |
| --- | --- | --- | --- |
|  |  | Years Ended December 31, |  |
|  | 2019 | 2018 | 2017 |
|  |  | (Dollars in millions) |  |
| Change in benefit obligation |  |  |  |
| Benefit obligation at beginning of year | $11,594 | 13,064 | 13,244 |
| Service cost | 56 | 66 | 63 |
| Interest cost | 436 | 392 | 409 |
| Plan amendments | (9) | — | — |
| Special termination benefits charge | 6 | 15 | — |
| Actuarial (gain) loss | 1,249 | (765) | 586 |
| Benefits paid from plan assets | (1,115) | (1,178) | (1,238) |
| Benefit obligation at end of year | $12,217 | 11,594 | 13,064 |
Paragraphs:
In 2019, 2018 and 2017, we adopted the revised mortality tables and projection scales released by the Society of Actuaries, which decreased the projected benefit obligation of our benefit plans by $4 million, $38 million and $113 million, respectively. The change in the projected benefit obligation of our benefit plans was recognized as part of the net actuarial (gain) loss and is included in accumulated other comprehensive loss, a portion of which is subject to amortization over the remaining estimated life of plan participants, which was approximately 16 years as of December 31, 2019.
The following tables summarize the change in the benefit obligations for the Combined Pension Plan and post-retirement benefit plans:
Question:
In which years was the revised mortality tables and projection scales released by the Society of Actuaries adopted?
Answer: 2019, 2018, 2017