16down voteaccepted
+50
Open the settings and navigate to Editor > General > Appearance and tick "Show hard wrap guide (configured in Code Style Options)".
Alternatively bring up the Find Action search with Ctrl+Shift+A (by default), search for "Show right margin" and hit enter to toggle the setting.
The trick is to change it with distraction free mode enabled.
EDIT: In versions of IntellJ before 2017.3, the menu option was called "Show right margin".