It's fleshing them out, definitely. The idea being that you start with a "skeleton" and then "add meat" to it.

Who on earth says "flushing out ideas?" That's worse than ambidextrious.

There is justification for using the noun "flush" in the context of ideas. New, rapid spring vegetative growth is called a "flush." In cards we have "flush" or five cards of the same suit. So, new ideas that grow from the bud or fill out a concept to make it complete could be referred to as a "flush." But as a verb, "flushing" doesn't fit my idea of the creative experience. I go with "fleshing out" a concept or idea.

If your idea is under heavy cover of brambles, you can scare it into the open so you can shoot it.

Then you're "flushing out" an idea.

Of course, when people talk about "flushing out" an idea, I often think about the more indoors version. Especially if it's a bad idea...