Aon (AON) revealed a 20% jump in fourth-quarter profit on Friday that just surpassed Wall Street expectations, as the world’s largest insurance broker continued to benefit from its lengthy overhaul and acquisition of Hewitt Associates.

While total operating expenses climbed 3% to $3 billion, charges related to its restructuring plan declined to $43 million. 

The company continues to benefit from synergies related to its 2010 $4.9 billion acquisition of Hewitt Associates, which created the biggest human resources services company in the world.

Aon, which recently revealed plans to move its corporate headquarters from the U.S. to London, said HR Solutions increased 3% to $1.2 billion on stronger commissions and fees.

"While macro economic conditions remain challenging globally, we are firmly on track to deliver improved growth in 2012,” Aon chief executive Greg Case said in a statement.

For the fourth quarter, the Chicago-based reinsurer posted net income of $277 million, or 84 cents, compared with a year-earlier $231 million, or 68 cents.

Excluding special items, the risk manager earned 97 cents, beating average analyst estimates in a Thomson Reuters poll by a penny.

Revenue for the three months ended Dec. 31 was $2.99 billion, up 3% from $2.90 billion a year ago, matching the Street’s view.

The company’s risk solution business saw sales climb 3% to $1.8 billion, while retail brokerage revenue grew 2% on demand in North America and Asia.

Despite the gains, investors seemed to yawn the news, sending Aon's shares down more than 2% to $48.31 Friday morning.

Follow Jennifer Booton on Twitter at @Jbooton