multi-state filing requirement

No. You do not have to file an Idaho tax return because you had no Idaho income.

Idaho says you must file individual income tax returns with Idaho if you're any of the following:

• An Idaho resident
• A part-year Idaho resident with income from Idaho sources or income earned while an Idaho resident
• A nonresident of Idaho with income from Idaho sources

Individual Income Tax

@ErnieS0 

Re-read?......they were Idaho residents most of the year, and in Utah only at the end...

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but  @bdblack6 

Which state did you actually live in between September and December?

@bdblack6   Utah requirements appear to be Very similar.

...except #1 seems to require a UT tax return if you have to file a Federal tax return.

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"You must file a Utah TC-40 return if you:

1. are a Utah resident or part-year resident who must file a federal return,
2. are a nonresident or part-year resident with income from Utah sources who must file a federal return, or
3. want a refund of any income tax overpaid.

To file a Utah return, first complete your federal return, even if you do not have to file with the IRS. You need the federal return information to complete your Utah return.

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Who Must File (utah.gov)

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Also, if you own Mutual funds, watch out for Year-End distributions that happen in early-late December....also month-end interest from any source that was 50cents or greater.  Those $$ would be UT part-year resident taxable income.