220 ILCS 5/7-206
The Commission may require every public utility engaged directly or indirectly in any other than a public utility business, as defined by law, to keep separately in like manner and form the accounts of all such other business, and the Commission may provide for the examination and inspection of the books, accounts, papers and records of such other business, in so far as may be necessary to enforce any provisions of this Act. The Commission shall have the power to inquire as to and prescribe the apportionment of capitalization, earnings, debts and expenses fairly and justly to be awarded to or borne by the ownership, operation, management or control of such public utility as distinguished from such other business. Provided, however, that an electric or gas public utility shall not be required to maintain the accounts of any non-public utility business in the same manner and form as the electric or gas public utility is required to keep the accounts of its public utility business unless expressly ordered by the Commission.
220 ILCS 5/7-206