Can I license an idea to a company without having it patented?

It would be a question of what is the idea actually worth - you may not know it, but an idea per se cannot be patented. Only an actual implementation of an idea - either as an apparatus, or method, or product, or the like, can be patented. Therefore, if an idea per se cannot be patented, one has to ask themselves what is an idea per se actually worth, and therefore can it be licensed? My guess would be only if the licensee actually sees or appreciates its value when actually implemented, but they would have to know or have some good idea of how to actually implement it.

Ummm ... it comes down to the basic rules of licensing ... how to detect cheating (on both sides) and what can be done to enforce the contract. Note that NDAs usually fall into trade secrets legislation and is completely separate from the licensing of IP (which can be based around copyright and right to access secret information). The trick with licensing is coming up with the right combination of licenseable material (if include patents need indemnities) and how you envisage the product to be developed to make sure the mutual advantages are retained ... see the Sun Java API v google clean-room cloning as counter-examples.