Idaho Code § 15-2-513

Whether or not the provisions relating to holographic wills apply, a will may refer to a written statement or list to dispose of items of tangible personal property not otherwise specifically disposed of by the will, other than money, evidences of indebtedness, documents of title, and securities, and property used in trade or business. To be admissible under this section as evidence of the intended disposition, the writing must either be in the handwriting of the testator or be signed by him and must describe the items and the devisees with reasonable certainty. The writing may be referred to as one to be in existence at the time of the testator’s death; it may be prepared before or after the execution of the will; it may be altered by the testator after its preparation; and it may be a writing which has no significance apart from its effect upon the dispositions made by the will.