To develop IntelliJ IDEA, you can use either IntelliJ IDEA Community Edition or IntelliJ IDEA Ultimate not older than 15.0. To build and run the code:

• Run getPlugins.sh / getPlugins.bat from the project root directory to check out additional modules.
• If this git repository is not on 'master' branch you need to checkout the same branches/tags in android and android/tools-base git repositories.
• Open the project.
• If an error notification about a missing required plugin (e.g. Kotlin) is shown enable or install that plugin.
• Configure a JSDK named "IDEA jdk" (case sensitive), pointing to an installation of JDK 1.6.
• Unless you're running on a Mac with an Apple JDK, add <JDK_HOME>/lib/tools.jar to the set of "IDEA jdk" jars.
• Configure a JSDK named "1.8", pointing to an installation of JDK 1.8.
• Add <JDK_18_HOME>/lib/tools.jar to the set of "1.8" jars.
• Use Build | Make Project to build the code.
• To run the code, use the provided shared run configuration "IDEA".

You can find other useful information at http://www.jetbrains.org. Contribute section of that site describes how you can contribute to IntelliJ IDEA.