The first step would be to Save As an idml (InDesign Markup Language) file, then create a fresh InDesign document by opening the idml. This will clear out any accumulated fragments of stale information in the document, and your new version should be noticeably smaller than the old one.
In each of the Character, Paragraph and Object Style panels, choose "Select all unused" from the panel menu and delete the redundant styles. If you see a warning message asking if you want to replace the style with [Basic Paragraph] or [None], choose Cancel, then repeat this step for that panel. Repeat until all the unused styles are gone with no warning messages.
If, after all this, you still get warnings about Type 1 fonts, see if there are any placed graphics (PDF, AI, EPS) that might be hiding them. You would want to replace these with versions that have OpenType or TTF versions of those fonts.
Repeat the Save As idml step to fully clear out any remaining crud, and you should have a good copy of the document going forward.