You do not have to reference a general idea or something that is common knowledge. However, any direct quotes, paraphrasing or rewording of another author's work will require citations. Likewise, if a fact is obscure, this too may warrant a citation.
As the Chicago Manual of Style (14.1) puts it:
"Ethics, copyright laws, and courtesy to readers require authors to identify the sources of direct quotations or paraphrases and of any facts or opinions not generally known or easily checked."
