All income is taxable to your home state. Some non-resident states prorate the income. Some, like MT, report the entire income and then prorate. You will have to file a tax return and possibly pay if you are over the filing requirement for MT. You might be entitled to a credit for taxes paid to another state if some or all the income taxed by Idaho is also taxed by another state.
Nonresident/Part-Year Resident Ratio Schedule (MT)
If you are a nonresident or part-year resident, you have so far reported your income as if you were a resident. You also calculated your tax as a resident on the Tax Liability Schedule, line 2. You must use this Schedule to reduce your income tax to the portion related to your Montana source income.
This is from 2018, but it was the most complete information I could find.
MT NR
