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Insurance broker Aon announced on Monday it will purchase rival USI Insurance Services from private equity firm KKR.

The $17 billion deal, which will be funded by Aon with new debt, is anticipated to close in the fourth quarter, subject to regulatory approvals.

CEO Greg Case in an appearance on CNBC's "Squawk Box" Monday said that the merger will establish the "premier U.S. middle-market platform."

"This means we're going to be in a position to bring world class solutions to the underserved U.S. middle market, and … set a new standard of client leadership for the 200,000 middle-market companies in the U.S. and their 48 million employees," he said.

The acquisition for Aon builds on the company's purchase of NFP in 2024, another insurance broker focused on the U.S. middle market.

Once a deal is closed, USI CEO Mike Sicard will transition to Aon's president and global CEO of middle market.

"Joining Aon represents a truly energizing next chapter for our firm and an opportunity to accelerate our momentum as part of the Aon United platform," Sicard said in the release. "Our firms share strong, one-firm cultures with a deep commitment to working together to bring the best of our capabilities to clients."

Piper Sandler insurance analyst Paul Newsome in an email said that insurance companies are interested in the middle market because it has unique growth.

"Middle market commercial generally growths a percentage or two faster than the large account commercial insurance business," he said. "I think the goal is to move their overall organic growth rates up a percentage point or two by shifting their business mix."

Shares of Aon tumbled 7% in response on Monday. But despite the initial slide, Case said the opportunity to serve the middle market at the scale the company now can through the acquisition has tremendous value potential for shareholders.

"Maybe the greatest I've seen in my 20-year career as CEO," he said.

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