Strong demand for shares in Willis led to an increase in the amount of stock to be made available in a public sale.
Interest from major institutional shareholders bumped up the number of shares on sale to nearly 22.2 million from an originally planned 16 million.
The shares will sell at $28.25 (£19.37). They were trading this week on the New York Stock Exchange at $30.50 (£20.91).
Majority shareholder Kohlberg Kravis Roberts will remain the company's largest shareholder with about 40% ownership of Willis.