Table:
|  | September 30, | September 30, |
| --- | --- | --- |
|  | 2019 | 2018 |
|  |  | (in millions) |
| Total backlog |  |  |
| Cubic Transportation Systems | $  2,953.3 | $  3,544.9 |
| Cubic Mission Solutions | 103.7 | 77.0 |
| Cubic Global Defense | 344.0 | 442.6 |
| Total | $  3,401.0 | $  4,064.5 |
Paragraphs:
Backlog
As reflected in the table above, total backlog decreased $663.5 million from September 30, 2018 to September 30, 2019. The decrease in backlog is primarily due to progression of work in 2019 on four large contracts awarded to CTS in fiscal 2018. In addition, we recorded a net decrease to backlog of $104.5 million on October 1, 2018 for the impact of the adoption of ASC 606. Changes in exchange rates between the prevailing currency in our foreign operations and the U.S. dollar as of September 30, 2019 decreased backlog by $79.7 million compared to September 30, 2018.
Question:
What led to the decrease in backlog?
Answer: primarily due to progression of work in 2019 on four large contracts awarded to CTS in fiscal 2018, impact of the adoption of ASC 606