What would cause car to over-heat after idling for 15 minutes?

It could be a variety of things, all having to do with the cooling system. Most likely it is a faulty engine fan that is supposed to automatically cut on when your engine hits a certain temp. It could the the fan motor itself, the relay that controls the fan, and maybe just a fuse. If the car does not overheat AT ALL in freeway driving (lots of cool air passing over the engine constantly) and overheats either idling or during city driving that engine fan is to blame.