If I can tell you're having a joke with another user and you post something like "lol! idiot, cya tomorrow mate :P" then I wont give an infraction as I can clearly see you're joking with the user.
It can seem harsh as "idiot" isn't a bad enough insult for an infraction really, a warning would be more suitable, that's why Elkaa is discussing it.
However, moron is worse than idiot. Sure, we use it everyday saying things like "agh why do that moron?" but the actual meaning of it isn't very nice.
It also means idiot, cretin, imbecile, a word that's filtered here, etc.
http://dictionary.die.net/moron