Axa SA Chairman and Chief Executive Officer Henri de Castries is leaving earlier than expected, handing over the post of CEO to Thomas Buberl as France’s largest insurer splits its top jobs.

De Castries, 61, will retire at the start of September, ending more than 16 years at the helm of Axa, the Paris-based insurer said in a statement on Monday. Denis Duverne, currently deputy CEO, will be chairman. De Castries’s mandate was scheduled to last until 2018. Buberl, a 42-year-old German, will move to Paris as deputy CEO this week to prepare for his new role at France’s second-largest financial firm by market value.

It was a surprise that de Castries stepped down, said Enrico Racioppi, an analyst at Hammer Partners SA in Lugano, Switzerland. “The company didn’t give any reason for the unexpected management change,” he wrote in a note to clients. “We see the news as short-term negative.”

Axa has grown into one of the world’s largest insurance companies under de Castries, building on its operations from France to the U.S. and Japan to expand in emerging markets such as China, Nigeria and Colombia. Axa’s net income rose 12 percent to 5.62 billion euros ($6.3 billion) in 2015 and the company said it met its five-year financial goals and will present a new strategic plan in June.

De Castries is the front-runner to succeed Douglas Flint as chairman of HSBC Holdings Plc, The Sunday Times reported, without saying where it got the information. HSBC appointed De Castries as a non-executive director in November. The bank plans to nominate a successor next year for Flint.

Axa has been seeking a successor since de Castries made a request that the board do so in October 2013, he disclosed in a letter to Axa workers. “Before we enter into a new strategic cycle, I considered, with the support of the board, that it was the best moment to begin the transition to a new management team,” de Castries wrote.

Buberl, formerly CEO of Switzerland at Zurich Insurance Group AG, joined Axa four years ago to lead its German business and became a member of the French insurer’s top-management committee last year. The company’s strategy is unlikely to change significantly under his leadership, said Benoit Valleaux, an analyst at Natixis SA in Paris who has a buy rating on the stock.

Axa has declined about 14 percent in Paris trading this year, the fourth-worst performance on the 36-member Stoxx Europe 600 Insurance Index. The stock was little changed at 21.85 euros as of 11:29 a.m.

De Castries took over the CEO job in 2000 from Claude Bebear, Axa’s founder, and added the chairman title six years ago. He joined the insurer in 1989 after working at the French finance ministry.

“From a corporate governance point of view, the market will like the fact that the company is splitting the roles of chairman and CEO,” Cave Montazeri, an insurance industry specialist at Barclays Plc, wrote in a note to clients. “I’d expect the stock to do well in the back-half of the second quarter as investors position themselves ahead of the June investor day.”