Bermuda reinsurer RenaissanceRe Holdings Ltd’s insurer financial strength rating has been upgraded to A+ from A by Fitch.The credit rating agency said the upgrade reflected RenRe’s continued strong leading position in the property/catastrophe reinsurance market and favourable underwriting results.The one-notch upgrade is a significant achievement for the company, especially in a market environment where reinsurance rates are falling.Fitch mentioned RenRe’s technical expertise, and particularly its catastrophe computer models, as being a strength that has helped it to effectively underwrite and price catastrophe-related risks.The ratings also reflect RenRe’s reasonable operating and financial leverage and overall high quality and liquid portfolio of fixed-income and short-term investments, although the company’s alternative investments expose its earnings to potential volatility.“RenRe has a history of generating strong underwriting profitability over extended periods of time,” Fitch stated. “Fitch views this as an important factor supporting the company’s ratings and as evidence of RenRe’s underwriting skills.“Fitch views RenRe’s year-to-year underwriting profitability as volatile, but the effect of this volatility on the company’s ratings is mitigated somewhat by RenRe’s low combined ratios over extended periods of time.“RenRe’s average overall calendar year combined ratio over the most recent 10-year period (2000-2009) was favourable at 73.2 percent with a standard deviation of 28.5 percent. For the first nine months of 2010, RenRe reported a calendar year combined ratio of 60.3 percent, the lowest ratio among peers.”