Idaho Code § 50-1724
Once bonds are issued as provided herein, any funds paid as instalment payments of assessments pledged to the payment of such bonds shall be kept in a fund known as the "bond fund" of the district and any funds paid as interest on said instalment payments of assessments shall be kept in a fund known as the "interest fund" of the district. The funds shall be deposited in such bank or banks as are designated as depositors of public moneys of such municipalities under the laws of this state, or invested in bonds or warrants of the municipality. Interest received on such funds so deposited or invested shall be placed to the credit of the fund from which it is earned. Maturing bonds shall be paid from the bond fund and the interest on the bonds, when due, shall be paid from the interest fund. If there is sufficient money in the bond fund to pay the principal of one or more bonds, the treasurer may call in and pay such bonds as of the next interest payment date in such manner as may be provided by the council at the time of the issuance of the bonds. The bonds to be called shall be selected by lot and shall, in the event less than all of the outstanding bonds are to be redeemed, insofar as can be done taking into consideration the denominations of the outstanding bonds, represent an equal amount of bonds from each maturity outstanding at the time of the redemption.
Idaho Code § 50-1724