There is a novelty requirement for US patents. Public disclosure of an invention negates its novelty. 35 USC § 102(a)(1):
A person shall be entitled to a patent unless the claimed invention was patented, described in a printed publication, or in public use, on sale, or otherwise available to the public.
If you disclose your invention, nobody (except you) can patent it.
If the inventor makes a public disclosure of an invention, that inventor can still file for a patent on that invention within one year of their disclosure.
A disclosure made 1 year or less before the effective filing date of a claimed invention shall not be prior art to the claimed invention [...] if the disclosure was made by the inventor [...]
Mere ideas can't be patented nor copyrighted. Patents protect new and useful processes, machines, manufactures, or compositions of matter, not ideas. Copyright protects expression, not ideas.