220 ILCS 5/5-102

The Commission shall have power to establish a uniform system of accounts to be kept by public utilities or to classify public utilities and to establish a uniform system of accounts for each class and to prescribe the manner in which such accounts shall be kept. It may also, in its discretion, prescribe the forms of accounts to be kept by public utilities, including records of service, as well as accounts of earnings and expenses, and any other forms, records and memoranda which in the judgment of the Commission may be necessary to carry out any of the provisions of this Act. Where the Commission has prescribed the forms of accounts to be kept by any public utility for any of its business, it shall thereafter be unlawful for such public utility to keep any accounts for such business other than those prescribed or approved by the Commission, or those prescribed by or under the authority of any other state or of the United States.

The Commission may, from time to time, alter, amend or repeal, in whole or in part, any uniform system of accounts, or the form and manner of keeping accounts.