Aon could get up to $500 million more based on future performance, as part of the deal.
London-headquartered Aon said it expected the deal to improve its return on invested capital and add to adjusted earnings per share in 2018.
Proceeds from the deal after tax are expected to be about $3 billion, subject to customary working capital and other adjustments, Aon said.
The company also said it expects to allocate part of the proceeds from this transaction to increase its share repurchases.
The repurchase program has been increased by $5 billion, bringing the total amount currently authorized for repurchases to about $7.7 billion as of Feb. 10, Aon added.
Reuters first reported the news on Thursday, citing sources who said Blackstone prevailed over buyout firm Clayton Dubilier & Rice in an auction for the deal.
The deal gives Blackstone ownership of a business that processes work benefits for 15 percent of the U.S. population.
Private equity firms have been keen investors in businesses that help companies cut costs by outsourcing large parts of their administrative functions, since such operations can generate strong cash flows.
They typically seek to sell ownership of such assets at a big profit a few years after they invest.
Aon also reported better-than-expected fourth-quarter earnings helped by strength in its retail business.
The company's net income attributable to shareholders fell to $502 million, or $1.87 per share, in the fourth quarter ended Dec. 31, from $584 million, or $2.09 per share, a year earlier.
On an adjusted basis Aon earned $2.56 per share, beating the average analysts' estimate of $2.49, according to Thomson Reuters I/B/E/S.
Morgan Stanley was Aon's financial adviser while Citigroup, Credit
Suisse, and SMB Capital advised Blackstone.
Sidley Austin provided legal counsel to Aon and Kirkland & Ellis to Blackstone.