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 December 31, | Year Ended December 31, | Year Ended December 31, | Year Ended December 31, | Year Ended December 31, |
| --- | --- | --- | --- | --- | --- |
|  | 2018 | 2017 | 2016 | 2015 | 2014 |
| Operating revenue | $44,438 | $41,138 | $39,450 | $40,704 | $40,362 |
| Operating expense | 39,174 | 35,172 | 32,454 | 32,902 | 38,156 |
| Operating income | 5,264 | 5,966 | 6,996 | 7,802 | 2,206 |
| Non-operating expense, net | (113) | (466) | (643) | (645) | (1,134) |
| Income before income taxes | 5,151 | 5,500 | 6,353 | 7,157 | 1,072 |
| Income tax (provision) benefit | (1,216) | (2,295) | (2,158) | (2,631) | (413) |
| Net income | $3,935 | $3,205 | $4,195 | $4,526 | $659 |
| Basic earnings per share | $5.69 | $4.45 | $5.59 | $5.68 | $0.79 |
| Diluted earnings per share | $5.67 | $4.43 | $5.55 | $5.63 | $0.78 |
| Cash dividends declared per share | $1.31 | $1.02 | $0.68 | $0.45 | $0.30 |
Question:
What was the operating revenue of Delta Airlines as of 2015?
Answer:
$40,704