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!