The Federal Energy Regulatory Commission (FERC) requires public utilities subject to FERC's jurisdiction and holding licenses issued by FERC to retain "copies of advertisements by or for the company on behalf of itself or any associate company in newspapers, magazines, and other publications, including costs and other records relevant thereto (excluding advertising of appliances, employment opportunities, routine notices, and invitations for bids all of which may be destroyed at option)" for 2 years.
Link to Regulation: https://www.ecfr.gov/cgi-bin/text-idx?SID=e48734063876363a29172bece2251a3d&mc=true&node=pt18.1.125&rgn=div5#se18.1.125_13