As of August 2017 and IntelliJ V2017.2, the accepted answer does not seem to be entirely accurate anymore: there is no fernflower.jar to use.
The jar file is called java-decompiler.jar and does not include a main manifest… Instead you can use the following command (from a Mac install):
java -cp "/Applications/IntelliJ IDEA.app/Contents/plugins/java-decompiler/lib/java-decompiler.jar" org.jetbrains.java.decompiler.main.decompiler.ConsoleDecompiler
(you will get the wrong Usage command, but it does work).