As far as I'm aware, all their IDEs are based on IDEA, almost acting as IDEA+special plugin, so a lot of code will still be the same. Of course, the language-specific stuff is not open, but as others have said, Java bytecode is fairly easily decompiled anyway.
Oh right, I always assume that JetBrains has the same strategy across its tools. Happened not for the first time. Thanks for reminding me!
I remember go being a plugin to intellij idea. Now it's not. Sadly. I wonder how they got over with this plugin having been opensource