Copyright pertains to when something, such as an idea, has been expressed in some form solid form - in writing, on video, mpg etc. Furthermore, copyright is part of Intellectual Property, to which there are all kinds of rights that could potentially protect your idea. Part of the problem here is defining what constitutes a unique "idea". If it is fairly loose - something that anyone could come up without needing to "steal" it from you - then there is no protection.
Eg (building on an earlier example) everyone wears shoes so you can't trademark shoes. But if you designed a unique shoe and you were clearly the source of that design, then you could trademark that. If you copied a Nike designed shoe they could successfully sue you (providing you are in a country that abides by these laws).
As such, is your "idea" is a vague idea, eg, this is a story set in world war II about a young solider having to leave his new wife... then it doesn't express anything unique enough to be considered your intellectual property.
But if your idea contains enough unique features that it would be virtually impossible for someone else to spontaneously come up with the same idea - and this idea must be recorded in some form (written, recorded etc) - then it will be protected by copyright as much at it would in a full script.
But copyright is not a guarentee that someone won't come out with a movie very similar to yours - legally or otherwise. While you might think your idea is incredibly unique, we are all influenced by the same world around us. Read the same newspapers, books, movies, music, etc - it is possible for someone completely unconnected to you to come up with a similar idea. Your copyright will not protect you from this - as they have equal copyright in their own work.
If someone reads your treatment/script and then goes off to develop something similar, then you can mount a court battle. It will come down to the "burden of proof". Registering copyright puts a proven date on it (providing you register it early on), additionally keep old drafts and notes, and details of everyone you share your idea with. In a court case you would need to prove that a) you came up with the idea first, and b) that person was exposed to your idea, and c) their execution uses unique elements of your idea to breach copyright.
But of course, it is hideously expensive to go to court. Even if you were completely in the right, how could you go up against a multi-billion dollar prodco? We just have to trust that people will act ethically, and not share our work with people we think might be unethical.