There is an official comment on the IntelliJ support site, stating the pre-2020.1 defaults per operating system are as follows:
- Mac OS:
Menlo - Linux:
DejaVu Sans Mono - Windows:
Consolas - Fallback:
Monospaced
I checked the default settings on IDEA version 2019.3 on OSX, and the default font is indeed Menlo, Size 12, Line spacing 1.2.
You can configure it by going to Preferences → Editor → Font