If you Google "Away beyond the hills in Idaho", you get references to Benny Goodman, among others...

If you Google "Benny Goodman Idaho" you get some sites with the song, and lots of other stuff that's not it at all. A lot of these lyrics sites seems to just copy nonsense from one another...

There is some consensus that he wrote it; whether that's a fact or a compounded error, I can't tell.