JetBrains Runtime is a runtime environment for running IntelliJ Platform based products on Windows, Mac OS XmacOS, and Linux. JetBrains Runtime is based on OpenJDK project with some modifications. These modifications include: Subpixel Anti-Aliasing, enhanced font rendering on Linux, HiDPI support, ligatures, San Francisco font family support on macOS, some fixes for native crashes not presented in official build, and other small enhancements.
This page has instructions on changing runtime version IDEs run under. Runtime version in use can be verified in IDE's About screen.
UPDATE Starting from 2017.1 version, 64-bit runtime is bundled on all platforms by default.