That's a tough one. Although you may be presenting your idea to an honest executive, most likely there will be a team of people that would get involved in the decision process. It is more likely than not the company would execute the idea without you, assuming, of course, your idea has merit and would pencil out. Not all companies are self-serving, however. Maybe you'll find the right one to speak with?
Ideas/concepts are not worth as much as a proven model.
Now, if your idea is such that you are the rare talent or have the secure connections required to execute, that's a bit different. Yet, even that is not a guarantee.
Is your idea able to be patented? Is it a widget, formula, etc?
I would consult a corporate attorney with your idea to see if he or she is able to help.