RLI Corp. tops the list of property/casualty commercial lines carriers ranked by financial performance for the 12 months ending March 31, based on a review by SNL.

RLI beat out Chubb, Dorinco Reinsurance and Argo Group for top honors.

RLI, a specialty lines insurer, is based in Peoria, Illinois.

Rounding out the SNL top 10 are Markel, Acuity A Mutual, Ally Financial, CUNA Mutual, W.R. Berkley and American International Group.

SNL ranked by financial performance 31 property/casualty entities designated as having a "commercial lines focus" and that held at least $500 million in policyholders' surplus as of Dec. 31, 2011. SNL designates a company as having a commercial lines focus if it writes the majority of its net premium in commercial lines business while not specializing in any single area of business, such as workers' compensation or medical professional liability.

SNL said it evaluated the insurers based on the following metrics, listed in order of importance: annualized rates of return, profitability, asset and surplus growth, investment performance, reserve development and premium growth. This analysis only includes NAIC (National Association of Insurance Commissioners) statutory filers.

To determine the specific performance scores, SNL calculated the mean among all of the companies for each of 13 selected metrics. SNL then calculated each company's deviation from each of the 13 means. The deviation from the mean was standardized using a Z-test so the different metrics chosen could be compared. The metrics were then categorized and a weighted average was applied to each category.

Returns, both on assets and equity, and profitability ratios relative to premium were the highest weighted categories at 35 percent and 30 percent, respectively. The rates of growth in assets and surplus reflect the annualized sequential change in each metric. To minimize the effect of outliers, SNL set a range limiting the amount of deviation recorded for each company.

SNL statutory insurance performance rankings