No... an idiom is a phrase does not mean what it literally translates to. Generally they arise from common speech or popular culture and become fixed in the language. An example in English would be "get out of here," or "get out of town." Literally, it translates to "go away," whereas, as an idiom, it means "you must be kidding me."
Slang is an informal register of speech. It is generally used between friends and varies widely between classes, regions, and cultural groups. It changes much more rapidly than the formal language, and tends to disregard some of the rules governing the standard, written form of the language. Often-times, this is to a degree intentional, as slang is usually spoken by youths or other groups that do not want to be understood... For instance, argot, a type of French slang, was first developed by criminal groups who were actively trying to avoid being understood by the authorities.