"Painting with a broad brush" is not an idiom, since it's meaning is deducible from its content, not only its contex. It's a metaphor (literally false as written, but calls to mind an analogy with truth -- both the comment and broad brushes apply the same thing to many targets without any careful differentiation)
Great link, thank you, I will use these words with more rigor in future. I think that “metaphor” is a much better word for the thing that is used to illustrate or add emotional oomph to an idea rather than to argue its merits.
http://learnersdictionary.com/qa/Idioms-metaphors-similes-an....