Yes, Will only provides the idea and him and him staff work together to come up with ideas based around this central idea, then they see what they can and can't (and cutting improbable ideas) do and make the best game possible utilizing as many ideas as they can.
When you think about it, though, without Will's mind we wouldn't have the basis for these ideas, at least not to be made into the games we know and love. Will has said he has other ideas for games, sure he might cut himself out of the game making process if he retires, but with how successful he is we'll still see games based on his ideas if he ever decides to sell them.
That or he could become a writer, which would probably kick some ass.