Adds details and background throughout

June 9 (Reuters) - W.R. Berkley WRB.N said on Tuesday that its founder and executive chairman William R. Berkley has died at the age of 80, marking the end of a long tenure that saw him transform the commercial insurer into a Fortune 500 powerhouse.

The company has appointed his son, W. Robert Berkley, Jr., as chairman, effective immediately.

William R. Berkley founded the company in 1967 and built it into one of the largest U.S. commercial property and casualty insurers. The company has a market value of about $25 billion, according to LSEG data.

W.R. Berkley operates a decentralized global business model comprising more than 60 specialized units that provide insurance tailored to specific industries and territories. It remains a major commercial insurer, consistently ranking among the top U.S. property and casualty providers.

Berkley was chief executive for nearly five decades before handing over the reins to his son in 2015 and becoming executive chairman.

"He was a father, best friend, and mentor to me and numerous colleagues, and his influence on us, our company, and the broader industry and the countless lives he touched and enhanced is immeasurable," Berkley, Jr. said in a statement.