There are IDL-specific editing modes for many of the more popular text editors:

IDLwave
This is the oldest and most complete mode for editing IDL code. JD Smith has been maintaining this for years. JD was looking for a new maintainer a while ago and moved IDLwave from its own website to GitHub, but everything seems to be still working.
BBEdit/TextWrangler
This mode includes a routine to scan IDL code and generate tags used for autocompletion.
Vim mode
Marshall Perrin has a Vim mode supporting IDL.
TextMate 2
I used TextMate 2 on the Mac and decided to write this simple mode that supports IDL code and IDLdoc comments. Ethan Gutmann had a TextMate bundle which I liked, but hadn’t been updated for TextMate 2 or newer versions of IDL.

Please let me know if you know of IDL modes for other text editors.

UPDATE 3/5: Some I missed that Jeff N. pointed out:

Notepad++
David Higgins maintains the IDL support for Notepad++.
Cream
Support for IDL is builtin!