Jay S. Fishman not only transformed Travelers Cos. Inc. during his more than a decade with the insurer, he also has had a major effect on the U.S. property/casualty industry in general.

Mr. Fishman, who has been CEO of Travelers since 2004 and has been diagnosed with a variant of Lou Gehrig's disease, announced earlier this month that he would step down as CEO on Dec. 1 but plans to remain executive chairman of the insurer.

Alan Schnitzer, currently CEO of Travelers' business and international insurance — the company's largest business segment — will succeed Mr. Fishman as CEO.

In a note to Travelers employees, Mr. Fishman said he has been dealing with a neuromuscular condition for some time.

“Given the progression of the symptoms I have been experiencing since informing all of you of my illness, it appears likely that I am dealing with a variant of amyotrophic lateral sclerosis, or ALS,” he wrote. “While this is a progressive disease, anyone who has seen me can tell you that I am more than able to play an active role in Travelers' future.”

Mr. Fishman served as CEO of the Travelers insurance operations during the time the insurer was part of Citigroup Inc., before joining The St. Paul Cos. Inc., where he was appointed CEO in 2001. In that role, he executed the 2004 merger of Travelers Property Casualty Corp. and St. Paul, with the resulting company known as Travelers.

Travelers declined requests for interviews with Messrs. Fishman and Schnitzer.

Howard Mills, currently global insurance regulatory leader at Deloitte L.L.P. in New York, knew Mr. Fishman when Mr. Mills was New York's superintendent of insurance.

“He will definitely have a long-lasting impact,” said Mr. Mills, who said he wishes Mr. Fishman “the very best.”

“He's built a massive company that's a market leader, very well-managed and a very strong company,” Mr. Mills said. “The merger that he engineered is probably the No. 1 thing that stands out and obviously had a very big impact on the market and will for the foreseeable future.”

Robert Hartwig, president of the New York-based Insurance Information Institute Inc., of which Travelers is a member, said in an email that Mr. Fishman's story is one of “consistent profitability, growth and stability — a rare combination in a volatile industry.”

He credited Mr. Fishman with successfully managing a turnaround of St. Paul in the “difficult post-Sept. 11 environment.” He followed that with the 2004 acquisition of Travelers.

“The company subsequently remained a standout performer during the global financial crisis. The company's strength and stability under Jay's leadership led to Travelers being named in 2009 to join the Dow Jones Industrial Average, replacing” Citibank Inc., Mr. Hartwig said.

Travelers remains the only insurer among the 30 companies that comprise the Dow Jones Industrial Average, Mr. Hartwig said.

Mr. Fishman “leaves the company in his capacity as CEO in excellent shape and in the very capable hands of his successor, Alan Schnitzer. The company remains poised to take advantage of future opportunities for growth, both domestically and internationally,” Mr. Hartwig said.

“They've done an awful lot of work building bench strength,” said Meyer Shields, managing director at Keefe Bruyette & Woods Inc. in Baltimore.

Regarding the market as a whole, Mr. Shields said Mr. Fishman did a “phenomenal job of advancing the utilization of analytics in pricing small commercial business, which has helped moderate the overall small-commercial pricing cycle.”

Leigh Ann Pusey, president and CEO of the Washington-based American Insurance Association, of which Mr. Fishman served as chairman in 2004 and 2011, said Mr. Fishman understood the “public policy imperative.”

Mr. Fishman “is comfortable walking the halls here in Washington as well as state capitals,” Ms. Pusey said. “He has an ability translate difficult technical insurance issues, which makes him such an asset for the industry. Whether it was 9/11 or the financial crisis or the issues in between, he was providing critical industry leadership.”