Idaho Code § 57-131
All deposits in public depositories shall be demand deposits or deposits in accounts upon which negotiable orders of withdrawal may be written, or in similar transaction deposit accounts except for deposits of surplus or idle funds which the said depositing units are authorized to make under section 57-127, Idaho Code, with the approval of their respective supervising boards. The term "surplus or idle funds" shall mean the excess of available moneys in the public treasury, including the reasonably anticipated revenues, over and above the reasonably anticipated expenditures chargeable to those moneys, taking into account the dates at which such revenues and expenditures may be expected to occur, the charges of expenses to revenues being done in such a manner as to produce the maximum amount of excess. This definition shall not apply to idle funds in the state treasury, which funds shall be as defined in section 67-1210, Idaho Code.
Idaho Code § 57-131