You have stumbled uppon the real problem behind intelectual property. That is nothing happens in a void. There are allways countless people that influenced and contributed to the idea
Yet, not many of these actually get their share of ownership. So it is NOT really the idea thats worth the protection, but the implementation (Sort of anyway) Or in case of patents the entities that undertake the process of patenting the idea.
So in essence you own the copyright, unless you worked as a employee or contract states otherwise, or some other exception. No matter how good the idea is their picture does not come to being without you.
This said you can still be reasonable. This is sane because the law is up to inerpretation. And here comes the disclaimer: I am not a lawyer, even if i was, I am not your lawyer.