A.M. Best Co. has placed under review with developing implications the financial strength rating of A (Excellent) and issuer credit rating of “a” of Essentia Insurance Company (Essentia) (Chesterfield, MO). The rating actions follow disclosure of a definitive agreement under which Markel Corporation (Markel) will acquire all outstanding shares of Essentia from its parent, OneBeacon Insurance Group, LLC, a subsidiary of OneBeacon Insurance Group, Ltd. [NYSE: OB]. The ratings will remain under review until the closing of the transaction and the completion of A.M. Best’s analysis of the impact the agreement will have on the company’s ratings. The transaction is expected to close during the first quarter of 2013, pending regulatory approval. The methodology used in determining these ratings is Best’s Credit Rating Methodology , which provides a comprehensive explanation of A.M. Best’s rating process and contains the different rating criteria employed in the rating process. Key criteria utilized include: “Risk Management and the Rating Process for Insurance Companies” and “Rating Members of Insurance Groups.” Best’s Credit Rating Methodology can be found at www.ambest.com/ratings/methodology. Founded in 1899, A.M. Best Company is the world's oldest and most authoritative insurance rating and information source. For more information, visit www.ambest.com. Copyright © 2012 by A.M. Best Company, Inc. ALL RIGHTS RESERVED.