You will file as a resident of Idaho. Your state of legal residency (SLR) is your “Home of Record,” unless
you changed it to another state. Your home of record is the state recorded by the military as your home
when you were enlisted, appointed, commissioned, inducted or ordered in a
tour of active duty.
From a tax standpoint, your State of Legal Residency (SLR) is considered your “domicile” or “resident” state as long as you are on active duty. Even if you are stationed in another state, you’re still considered a resident of your SLR.
You can read more on this subject here: What is my military state of residence?
As a reservist, you are not bound by the same "HOR/SLR" rules as active component members. You should speak to your finance department in order to update your current HOR, as you have probably been paying taxes to the wrong state this whole time.
So question pertaining to this. I enlisted in Oklahoma in 2014, moved to Virginia in December 2017, worked in Virginia all of 2018, changed my SLR to Virginia but left my HOR Oklahoma. When I filed my taxes it autogenerated as Oklahoma since that is what is on my DFAS record, but my resident address as Virginia's address. It autogenerated to Virginia as a non-resident. Is this accurate? Do I file it as Oklahoma Resident, Virginia non-resident?
I received a notice that I owed Oklahoma taxes as my Virginia civilian job did not take out Oklahoma taxes, so I am having to go back and amend my 2018 return.
By the way....I'm in the reserve, I just keep Oklahoma as my HOR for retirement purposes.
Your active duty military pay can only be taxed by your SLR, regardless of where you are stationed. Your SLR and your HOR are two different things, as explained here:
https://www.military.com/money/personal-finance/taxes/home-of-legal-record-for-taxes.html
Thus, if you formally changed your military SLR during 2018 by filing DD Form 2058, then you should have filed your 2018 returns as a part-year resident of each of the two states.
Civilian (non-military) pay earned by you in a non-SLR state would be taxable to you as a non-resident by that state. It would also be taxable by your SLR, but in that situation you'd be able to take a credit on your SLR state return for the taxes paid to the non-SLR state on the income taxed by both. Civilian pay earned in your SLR would be taxable by your SLR.