Idaho Code § 31-4106
The petition shall be filed with the county clerk and recorder of all counties in which the signers on the petition are located. If the petition is filed with more than one (1) county clerk and recorder each petition shall state the number of signing petitioners and the name of the county where the greater number of petitioners reside. Upon the filing of the petition or petitions the county clerk and recorder shall examine the petition and certify whether the required number of petitioners have signed the petition. In the event more than one (1) county is involved, the county or counties that have the fewer number of petitioners shall transmit the petition to the county clerk and recorder of the county containing the greater number of petitioners. The county clerk and recorder in the county containing the greater number of petitioners, shall, within thirty (30) days following the receipt of the petitions, transmit the petitions to the board of county commissioners of the county in which the greatest number of petitioners reside, together with his certificate, and the certificates of any other county clerk and recorder as to the sufficiency of the petition.
Idaho Code § 31-4106