INDD Opening In Illustrator

Every time I save an InDesign file, and then reopen the file, it will open in Illustrator instead of Indesign. I can override with the "open with" function, but I am not sure why this keeps happening. I uninstalled and redownloaded both programs and have started new files from scratch but my INDD files keep opening in Illustrator by default. Not sure if anyone else has ran into this problem or if its a super easy fix I am missing.
