Aon could be liable for more than the $50m it set aside to settle state investigations into its broker practices.
The company said it's own investigations had not dug up any evidence of bid-rigging or fraud as alleged by the New York attorney general Eliot Spitzer.
Investors reacted positively to the announcement and share prices rose by $1.97 to close at $24.52 on the New York Stock Exchange on Tuesday.
Aon's chief financial officer, David Bolger, said the $50m the company has designated for settling Spitzer's suit could increase.
"It's what we think is the appropriate number at this time, but it could change because we don't have a settlement," he said.