NEW YORK—Standard & Poor's Corp. on Thursday raised its financial-strength rating of Arch Capital Group Ltd.'s insurance and reinsurance companies to A+ from A.

S&P adjusted its rating of Bermuda-based Arch based on its “strong management and well-conceived corporate strategy.” Moreover, S&P analysts said Arch's strong track record of underwriting and operating performance has been better and less volatile than that of other Bermuda-based insurers and reinsurers.

The outlook on all Arch companies is stable, S&P said in a statement.

Analysts did note that the ratings could come under pressure if the company's underwriting performance struggles, earnings volatility increases over a multiyear period or if there is a major change in Arch's investment portfolio.

The New York-based rating agency said it does not anticipate raising its ratings on Arch during the next two years.