I have an invention idea for stopping drunk driving but not sure if an 'idea' is a patentable thing.
Inventions are patentable 35 USC 101:
Whoever invents or discovers any new and useful process, machine, manufacture, or composition of matter, or any new and useful improvement thereof, may obtain a patent therefor, subject to the conditions and requirements of this title.
I think you might be using the word "idea" in a more colloquial sense, though. Your "idea" might be a patentable invention, dependent on whether it is new, useful, and directed to one of the subject matter areas listed above.
Whether your description is more than just an idea probably turns on whether you can satisfy the enablement requirement. 35 USC 112(a), etc. That is, can you describe your invention/idea in sufficient detail such that a person skilled in the art could make and use it?
You can run a search on free patent databases (google patents, espacenet etc.) first to see if others have already come up with an invention with your idea or not. If not, you can file a provisional application which serves the purpose of getting a filing date. It is a bare bones application but within one year you have file an application for your worked out invention. The latter application does not reflect an idea but the invention while provisional application may reflects an idea only.