All copyright works have their basis on an idea. But Copyright cannot protect intangibles. When the author or the creator decides to use certain modes of expression such as writing or painting to expose that idea then Copyright only protects the way in which those ideas are expressed. It is that expression that makes a work original.
This also means that there can be many different works about the same idea and all of them will be protected by copyright regardless of its quality as long as they express this idea in an original and tangible manner. For example more than one person may have an idea about writing a book on a boy who rose from rags to riches but all of them will have different characters in the book or the story may take place in different places.