D.C. Code § 34-1109: Uniform accounts to be rendered; separate account of other business may be required.

Every public utility shall keep and render to the Commission, in the manner and form prescribed by the Commission, uniform accounts of all business transacted. Every public utility engaged directly or indirectly in any other business than that of the conduct of a street railway, or the production, transmission, or furnishing of heat, light, water, or power, or the conveyance of telegraph or telephone messages, shall, if required by the Commission, keep and render separately to the Commission in like manner and form the accounts of all such other business, in which case all the provisions of this subtitle shall apply with like force and effect to the books, accounts, papers, and records of such other business.