Idaho Code § 61-525
The commission shall have power, after hearing, to require any or all public utilities, except such as are subject to the act of congress entitled "An act to regulate commerce," approved February 4, 1887, and the acts amendatory thereof and supplementary thereto, to carry a proper and adequate depreciation account in accordance with such rules, regulations and forms of accounts as the commission may prescribe. The commission may from time to time ascertain and determine and by order fix the proper and adequate rate of depreciation of the several classes of property of each public utility. Each public utility shall conform its depreciation accounts to the rates so ascertained, determined and fixed, and shall set aside the moneys so provided for out of the earnings and carry the same in a depreciation fund and expend such fund only for such purposes and under such rules and regulations, both as to original expenditure and subsequent replacement as the commission may prescribe. The income from investments of moneys in such public fund shall likewise be carried in such fund.
Idaho Code § 61-525