Idaho taxes non-residents only on Idaho-source income.
This document from the Idaho State Tax Commission gives its definition of Idaho-source income:
https://tax.idaho.gov/aguideidsourceincome.pdf
Clearly the Idaho Tax Commission does not consider work performed in Washington State to be Idaho-source income.
Note that the document states that you or your employer can calculate the Idaho compensation percentage.
With regard to previous years, you must file an amended Idaho return within three years from the due date of the original tax year return, including valid filing extensions.
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
