Realmente eres un idiota.

I’ve never heard “idiota” used to mean jerk, as far as I’m aware it means essentially the same thing as “idiot” in english, but I could be wrong.

I think the translation is a polite version of the original. My dictionary defines jerk as:

2 (contemptible person) (informal) estúpido m/ estúpida f ; memo m/ mema f (informal); pendejo m/ pendeja f (Latin America excluding Southern Cone) (informal); gilipollas mf (Spain informal); huevón m/ huevona f (Andes, Venezuela informal)