I created my own .desktop file because I've moved the folders where IntelliJ IDEA was installed.
[Desktop entry]
Name=Intellij_IDEA
Exec=/home/jorch/Documents/1.IDEsAndPrograms/idea-IU/bin/idea.sh
Icon=/home/jorch/Documents/1.IDEsAndPrograms/idea-IU/bin/idea.png
Type=Application
Comment=Integrated Development Environment
NoDisplay=false
StartupNotify=true
Categories=GNOME;Development
X-Desktop-File-Install-Version=0.22
Here is what's inside my intellij.desktop in /usr/share/applications.
I have even checked if there were some "old files" as suggested here.
I also tried, login out or running Alt+f2 and then unity to restart unity.