A.M. Best Co. has revised the outlook of Principal Financial Group Inc. and two of its life insurance operating companies to stable from negative. The financial strength rating of “A+” and issuer credit ratings of “aa-” for Principal Life Insurance Co. and Principal National Life Insurance Co. also were affirmed by A.M. Best, as was the “a-” issuer credit rating of their parent company. However, A.M. Best said Principal could realize additional losses on its $4.3 billion commercial mortgage-backed securities portfolio and its $8.8 billion direct commercial mortgage loan portfolio.