Fine.
Wikipedia:
idiom: an expression, word, or phrase that has a figurative meaning that is comprehended in regard to a common use of that expression that is separate from the literal meaning or definition of the words of which it is made
Dictionary.com:
idiomatic: peculiar to or characteristic of a particular language or dialect
Merriam-Webster:
idiomatic: peculiar to a particular group, individual, or style
Take your pick. They all say roughly the same thing.