Global reinsurance and insurance broker Willis Group Holdings and professional services and analytics firm Towers Watson have agreed to an all-stock merger of equals valued at $18 billion.

Upon completion of the merger, Willis shareholders will own approximately 50.1 percent and Towers Watson shareholders will own approximately 49.9 percent of the combined company.

The combined company will be named Willis Towers Watson and be domiciled in Ireland, where Willis Group moved its place of incorporation, previously Bermuda, in 2010.

Willis Chairman James McCann will be chairman of the combined company, while Towers Watson CEO John Haley will be CEO. Dominic Casserley, current Willis CEO, will be deputy CEO of the merged firm. Haley and Casserley will serve on the board, along with six Willis directors and six from Towers Watson. Towers Watson’s Roger Millay will be chief financial officer

The transaction has been unanimously approved by the board of directors of each company.

The combination brings together two complementary businesses to create a global advisory and brokerage firm. The combined company will have approximately 39,000 employees in more than 120 countries and revenue of approximately $8.2 billion.

The announcement said the combination is expected to result in $100-$125 million in cost savings within three years of closing, primarily related to the elimination of duplicate costs and economies of scale, in addition to increased efficiencies.

The firms see Towers Watson’s relationships helping to increase Willis’ penetration in the large U.S. property/casualty insurance corporate market and Willis helping to expand Towers Watson’s business internationally.

“The rationale for the merger is powerful—at one stroke, the combination fast-tracks each company’s growth strategy and offers a truly compelling value proposition to our clients,” said Willis CEO Casserley. “Together we will help our clients achieve superior performance through effective risk, people and financial management.’

He said the combined firm will advise 80 percent of the world’s top 1,000 companies, as well as having a significant presence with midmarket and smaller employers around the world.

Towers Watson CEO Haley said the deal will also mean savings for middle-market clients and increased development opportunities for employees.

Casserley and Gene Wickes from Towers Watson have been chosen to oversee the integration.

Willis has been growing geographically, completing more than a half-dozen deals since January of 2013, when Casserley took over as CEO. Casserley said last October that his acquisition strategy focused on specialty businesses that have “strong franchises,” rather than going after volume. In January, it took a majority interest in London broker Miller Insurance Services. In April, Willis offered to buy the 70 percent of Gras Savoye that it didn’t already own to expand in France, Eastern Europe and the Middle East. Also, in October of last year Willis invested $205 million in Swedish personal lines broker Max Matthiessen.

At the same time that it has been acquiring, Willis has been cutting expenses by laying off and relocating employees.

The transaction is expected to close by Dec. 31, 2015, subject to customary closing conditions, including regulatory approvals and approval by both Willis and Towers Watson shareholders.

Yesterday, Willis announced that Deputy CEO Steve Hearn was leaving to become CEO at Cooper Gay Swett & Crawford at the beginning of November. Willis executives said Hearn knew about the pending merger and supported it.

Last month, Peter Hearn, chairman of Willis Re, left to join Guy Carpenter as its CEO beginning in May 2016.