Can you patent an idea? Not usually, but, yes, technically, with constructed reduction to practice and only if the idea is patent eligible subject matter, useful, novel, non-obvious, and a complete and proper patent application is filed and allowed; then, yes, you can patent an idea for an invention without having the actual invention. Patenting an idea is not a recommended best practice, but it is possible. 

Actual reduction to practice is not required. In other words, you do not have to actually make the invention in order to be able to apply for and be allowed a patent. However, it must, in fact, be actually possible to make it. Importantly, while actual reduction to practice is not required, constructive reduction to practice is required. 

You can potentially get a patent for an idea, but constructive reduction to practice is required. In other words, when you are seeking to patent an idea you must be able to completely describe how to make and use the invention. The written description must include enough detail and instruction to teach a Person Having Ordinary Skill In The Art (PHOSITA) how to be able to make the idea. Of course, there can be new ways (means and/or methods) used to make the idea, but they cannot be speculative they must be described. 

Can You Patent an Idea Without a Prototype?

Can you patent an idea without a prototype? Yes, in fact, prototypes are not required. Unless the invention is a perpetual motion machine, prototypes are not part of a patent application. However, creating prototypes is a valuable and potentially very important activity. Development and engineering of prototypes frequently offers insights into the idea/invention that includes patentable subject matter.