What if I moved to Idaho and then back to Utah in six months, but I usually live in Utah

"Domicile" is the place you call home. It is the place where you bought a home or signed a lease, where you receive your mail, where you have a landline (if such things still exist), where you signed up for your drivers license, where you registered to vote, and perhaps most importantly the place that you tell friends and bosses where you live.

This is where you are a "resident", and you don't stop being a resident unless you take positive steps to relocate elsewhere.

People often leave their domicile for temporary reasons, like go to school out of state, do a temporary job out of state, and so on. But this doesn't change your domicile.

From your description, your resident state is Utah, and you would need to file a non-resident return for Idaho. Idaho gets to tax you because you worked there, but Utah gets to tax you because it is your domicile (and you also probably work there).