Because of this they will want to know about it. Many organisations have a Company Suggestion Scheme to provide a ready path for good ideas to be considered and rewarded. Depending on scheme rules it is normal for an award to be given based on a percentage of projected savings in exchange for allowing your company to use the idea. That does not necessarily mean you have to give up ownership rights to the idea - merely that you have in effect given a licence to the company to benefit from its use in exchange for an award. Your scheme rules should make the position clear.
Awards based on potential savings are normally tax free.
Remember not to disclose your idea outside the company before consideration on patentability is given. If your employer is interested in having a prototype made, any approach to an outside contractor should be made under a confidentiality agreement to avoid the risk of invalidating any subsequent patent. Keep a record of what is provided including sketches. Large companies often have an intellectual property department, and a patent attorney from that department can advise on these matters. Alternatively, your company may have set up links with an external patent attorney.