Idaho Code § 61-1004
On or before April 15th of each year the commission shall determine the proportionate assessment that all railroad corporations, and all other public utilities subject to the jurisdiction of the commission, shall bear to the amount which will be required to defray the expense of the commission for supervision and regulation of such railroad corporations and other public utilities during the ensuing fiscal year; such determination shall be based upon a consideration of the time and expense devoted to the supervision and regulation of each such class of railroad corporations and other public utilities during the preceding calendar year, including salaries and wages of the commissioners and employees and all other necessary and lawful expenditures of the commission. Thereupon the commission shall apportion the assessment thus determined to be required of all railroad corporations and all other public utilities, to each such class thereof, respectively, in proportion to their respective gross operating revenues derived from intrastate utility business in Idaho for the preceding calendar year, except that the maximum fee payable shall not exceed:
Idaho Code § 61-1004