I am not sure if it caould be called an idiom, but the phrase 'well off' or 'better off' always stimulates my curiosity. IMO 'well' could refer to healthy or in a good way while 'off' suggests a separated state. Could you please tell me what the picture is for 'well off' meaning richly or something like that?
Sorry, but this meaning of off doesn't really match up to any of its other meanings. Maybe one of the other teachers in another time zone will have the energy to do the etymological research and tell us both why.