Idaho Transfer is a 1973 science fiction time travel film directed by Peter Fonda and starring Keith Carradine (I loved the TV series Kung Fu in which he played the main character as a teen). I tend to dislike time travel movies.
Does the claim that this movie was filmed entirely in Idaho serve as evidence of the actual existence of Idaho? Just askin'.
It can be watched online from the Internet Archive:
Million Monkey Theater calls it "a weird ass movie" and has a lengthy plot description.