An idea is a general notion or conception formed by generalization, and according to some philosophers example, Plato; an idea is an archetype of which a corresponding being in phenomenal reality is an imperfect replica. According to Kant; It is a concept of reason that is transcendent but nonemprical, and for Hegel; it is an absolute truth, the complete and ultimate product of reason.
For Immaterial, I may say little of it, It is not formed of matter, it is incorporeal, it is spiritual, have no material body or form

Can you develop your question a little? Do you have any particular philosopher (or theory) in mind?

I believe the difference is simply that Idea can denote a Concept, Judgment, Principle, or deduction, grounded on either empirical or non-empirical evidence. For instance: the idea of change, as in the change one goes through from birth to the end of life, or the idea of universal change, as in the change all things within the universe, including the universe itself, undergo. This would be a universal idea of change.

The word, immaterial, can denote anything without physical substance. Thus, ghosts, spirits, or anything that Kant would call noumena ( outside of the world of phenomena).

So Idea: A thought, or a judgment, or principle, etc.
And Immaterial: Noumena.

But noumena can also lead to Ideas. And Ideas may shed some light on noumena.