idiom vs manner

idiom and manner both are nouns.

Word	Noun	Adjective	Verb	Adverb
idiom	Yes	No	No	No
manner	Yes	No	No	No

As nouns, manner is a hypernym of idiom; that is, manner is a word with a broader meaning than idiom:

Other hypernyms of idiom include fashion, mode, style, way.

idiom (noun)	manner (noun)
the style of a particular artist or school or movement	a way of acting or behaving
the usage or vocabulary that is characteristic of a specific group of people	how something is done or how it happens
an expression whose meanings cannot be inferred from the meanings of the words that make it up	a kind
a manner of speaking that is natural to native speakers of a language