Home directory for bundled version of JDK in IntelliJ IDEA
If you want to know, where the bundled JDK is, try this:
[Ctrl+Shift+A] within IDEA and type "JDK" and select "Switch IDE Boot JDK"
You should see the bundled directory and you can switch to an other JDK path.
Please be aware that Java 8 is required to run IntelliJ IDEA starting from version 2016 on all the supported platforms.
EDIT:
Starting with version 2019.1:
[Ctrl+Shift+A] within IDEA and type and select "Choose Runtime..." (requires the 'Choose Runtime' Plugin)