OLDWICK, N.J.—Ratings agency A.M. Best Co. has issued a flurry of end of year ratings actions, affirming a number of insurers, upgrading one and downgrading another.

The Oldwick, N.J.-based Best has decided to leave the A+ financial strength ratings of Munich Reinsurance Co. unchanged following the announcement earlier this week that the German reinsurer and insurer planned to buy HSB Group Inc. from American International Group, Inc.

The planned purchase of HSB Group, which owns machinery/plant and equipment breakdown insurer The Hartford Steam Boiler Inspection and Insurance Co., is a further step in Munich Re's strategy for the U.S. insurance market, Best noted in a statement.

Best also upgraded one of Munich Re's Canadian insurance subsidiaries, Temple Insurance Co., to A+ from A. The outlook for the rating has been revised to stable from positive.

Best said that the rating of Temple Insurance Co., a niche provider of liability and commercial property coverage, reflected the insurer's capitalization, consistently solid operating performance and benefits from its strategic affiliation with Munich Reinsurance Co. of Canada and its ultimate parent, Munich Reinsurance Co. in Munich, Germany.

In an unrelated move, A.M. Best also announced that it has downgraded the financial strength ratings of Cooperative Mutual Insurance Company to B+ from B++ and has revised the outlook to negative from stable.

Cooperative Mutual Insurance Co. specializes in writing property/casualty coverage for farmer-owned agricultural cooperatives in the United States. The rating downgrade reflected the significant deterioration in risk-adjusted capitalization during 2008—which were generated from investment losses associated with the current economic downturn—Best said in a statement.

In other rating moves, Best affirmed the A++ financial strength ratings of Chubb Corp. and assigned an A financial strength rating to Starr Insurance & Reinsurance Ltd. (Bermuda), a wholly owned subsidiary of Starr International Company, Inc.