“But I'm equally excited about the path ahead for Willis under the leadership of Dominic Casserley and Steve Hearn and our outstanding team of leaders, and the 17,000 associates around the world,” he adds in a statement. “In the past and in the future, it is our people who provide expertise, uphold our values and put a human face on The Willis Cause for every one of our clients.”

This year Plumeri was named one of NU's “Living Legends of Insurance.” Find our story here.

Speaking of his leadership style, Plumeri in June told NU, “The first thing you learn about leadership is that it is not about you.”

“Leadership is just the opposite of what people believe,” he adds. “If you are a real leader, you appreciate the fact that the people you have the good fortune to inspire and the people you have the good fortune to motivate are the people who give you the opportunity to work with them, and it's these same people who make the success of the company, not you.”

Plumeri saw the company's net worth double over his first eight years. As the world faced a financial crisis in 2008, Willis acquired brokerage firm Hilb, Rogal & Hobbs in a $2.1 billion deal to firmly establish its presence in the United States while it expanded in Europe, Asia and Latin America. Last summer Willis opened its 22nd office in China, where it was the first brokerage to get a license to operate.

Casserley, 54, now based in London, will move to Willis' office in New York City after a transition period in 2013.

During his time with McKinsey, Casserley has been based in the U.S., Asia and London. Now a senior partner at the firm, he led McKinsey's Greater China Practice and its UK and Ireland Practice. Willis says Casserley is an expert on global financial services, including insurance companies. He has written two books on the subject.