I set up my IDEA environment by doing the following:
Folder, toss the contents of the forge source zip in it, delete the example mod
gradlew setupDecompWorkspace
gradlew idea
When I go to open a class from Vanilla or FML, but not forge, it warns: Source not found, and puts /* compiled code */ in place of the code I want to read.
The code from both appears to be deobfuscated, however not decompiled.