Jay Fishman, who engineered the merger between St. Paul Cos. and Travelers Property Casualty then steered the insurer through the financial crisis and into the Dow Jones industrial average, is stepping down as chief executive officer because of his health.

“It appears likely that I am dealing with a variant of amyotrophic lateral sclerosis, or ALS,” Fishman, 62, said in a letter to staff.

Alan Schnitzer, 49, who leads the unit selling business and international insurance, Travelers Cos.’ largest segment, will become CEO Dec. 1, the New York-based company said in a statement.

In November, Fishman said that he’d been diagnosed with a neuromuscular condition, without specifying the ailment. He later said he made the disclosure to provide clarity to people around him who had noticed that “something was amiss.”

At every company, “there comes a time for new leadership,” Fishman said in the letter. “Because of the ­progression of my neuromuscular condition, this time has come a little earlier than I had hoped.”

In an era of Wall Street excesses and irresponsible financial behavior, Fishman stood out for his decency and restraint, Forbes reported.

“It is important to me that I let each of you know that I get way too much personal credit for the success we have experienced together,” Fishman told Travelers’ employees on Tuesday. “The credit for our success is to be shared by everyone here.”

Schnitzer has been one of Fishman’s top deputies at Travelers. He joined the insurer in 2007 and helped expand operations in Canada in 2013 with a deal to buy a business from E-L Financial for more than $1 billion.

Schnitzer serves on the U.S. Securities and Exchange Commission’s Investor Advisory Committee and is a member of the Council on Foreign Relations, according to the statement.

Bloomberg news