Why does USAA have an F rating?

American Fidelity Assurance 21 Bright Health 27 Centene Corporation 32 Federated Mutual Insurance Company 12 GAINSCO 13 Independence Blue Cross 21 Kemper Corporation 17 Medical Mutual of Ohio 25 Mercury Insurance Group 19 Missouri Employers Mutual 15 Mutual of Omaha 17 Nationwide Mutual Insurance Company 16 New York State Insurance Fund 28 Oxford Health Plans 18 ProSight Specialty Insurance 20 Prudential Financial 29 Sun Life Financial 23 Westfield Insurance 12 XL Catlin 23 Zenith Insurance Group 13