Ideas cannot be copyrighted, only the EXPRESSION of ideas.
Federal statutory copyright comes into existence when you fix the idea in a tangible medium of expression. However, you gain additional rights by placing a copyright notice on it (either by placing the word "copyright", or the c in a circle symbol, your name and the year in which you fixed it in a tangible medium of expression) and when you register the work with the Register of Copyright -- whose office is located in the Library of Congress. You can get the forms to register your copyright online at www.copyright.gov/register.
You will still face risks. When you write down your idea (writing is the most common form of "fixation in a tangible medium of expression") your writing is protected, but not necessarily your idea. For example, you can copyright your idea for an airplane dogfight game, but this does not stop others from developing their own dogfight game using the same general idea, but an expression of that idea that is different than yours. Mere ideas are not a subject for copyright -- but specific fixed expressions, such as writings, or drawings or a combination of both are.
Patent protection is sometimes available as well.
But copyright is faster and far less expensive.
Actually, there is one way to protect your idea, which is by contract. If you can get the company that you want to pitch your idea to, to agree that they will pay you the reasonable value of the idea that you are about to disclose to them BEFORE you disclose it, that will work. The drawback in this method is that none of the companies that you would like to pay you $1,000,000 will agree to such a procedure. If you could get them to agree, though, the contract method would work.
A registered copyright is better protection than not having one. Good luck.
www.copyright.gov; the treatise NIMMER ON COPYRIGHT.