Please read the following table in Markdown format and then answer the question according to the table. Table cells in one row are seperated by '|', and different rows are seperated by '
'.

Table:
|  |  | Year ended | Year ended |  |
| --- | --- | --- | --- | --- |
|  |  | December 31, | December 31, |  |
|  |  | 2019 | 2018 | Percent Change |
| Operating income |  | $2,957 | $3,206 | (7.8) |
| Net income |  | $2,300 | $2,465 | (6.7) |
| Net income per share, diluted |  | $4.27 | $4.29 | (0.5) |
| Non-GAAP | Operating income | $2,957 | $3,167 | (6.6) |
| Non-GAAP | Net income | $2,300 | $2,435 | (5.5) |
| Non-GAAP | Net income per share, diluted | $4.27 | $4.24 | 0.7 |
Question:
What was the percentage change in GAAP operating income of Southwest from 2018 to 219?
Answer:
(7.8)