I have been using Nova as my main text editor for the last few months. It is a Mac-only, extendable editor that has a great "remote mode" which allows me to edit files on remote servers from my laptop. There are many extensions for handling various languages and data formats, but no IDL extension. So I created a simple one. It is definitely not a complete language specification that makes use of all of Nova's extension capabilities, but it does basic syntax highlighting and some code completion.
You can download the extension through the app — just search for "IDL" in the extension library. Check out the source code and contribute to the project on the GitHub repo.