Idea remains a somewhat unclear concept, so it is important to be aware of both the context in which it is used, and the form of “idea” Hegel is employing. Hegel often uses it nearly interchangeably with "Spirit." At one point in Introduction to the Philosophy of History, he refers to “idea” as lying in "the innermost pit of Spirit." In general, he uses “idea” in the context of a summarized, effective form of the very loose concept of Spirit—almost as a practical, active version of Spirit.
The “idea” is what directly informs the universal principle of the State in its many forms, and when Hegel is discussing Reason, he often expands the term rational idea to imply that Reason is not only an abstract concept but also a driving force in human history. “Idea” is also referred to as something Spirit has, as the thing it wants to realize in the world. This usage only points out the extent to which Idea and Spirit overlap, since Hegel also says that Spirit only seeks to realize itself.