LOL! why Idaho or Montana. Why not any other state?
I just imagine they have a lot of land that is undeveloped. I mean the criminals can still work, hell it would be doctors, lawyers, teachers, dentists and everything there.
It'd be a mirror of what you have in your state eventually unless some thugs took over Taliban style. Then they might have to go to "real" jail.
Initially, it would be to keep them away from "regular" citizens.
Rappers will be there too. Actors ... all kinds of people.
Hell, you'd be able to take a safari style trip through there every now and then... at your own risk.
I think it would be better than locking people up, if you could show that you have been rehabbed... you'd get to leave after your sentence is up.
Some people might even stay.
It wouldn't be a free for all, there would be police there too to take the real nutcases off of the streets. People would try to function the way they did before... just not around regular people but people that bent the rules.
Murderers and rapist and all of that... would be sent to Utah.