Even for those of us who had been questioning the opaqueness and corporate governance at American International Group (NYSE:AIG) for some time, the disclosure today that Maurice "Hank" Greenberg was planning to step down as chief executive is a bit of a shock.

Greenberg has sat in the CEO's chair at AIG for nearly 40 years. In that time he guided AIG's change from an also-ran insurer with a great franchise in China to one of the world's financial powerhouses, a Dow Jones Industrial Average component company with three-quarters of a trillion dollars in assets and more than a trillion in assets under management.

AIG is an embodiment of Hank Greenberg -- hard-nosed, sharp-elbowed, and ruthless. Greenberg is known for, among other things, bawling out Wall Street analysts as "know-nothings" if they dared lower ratings or otherwise questioned the company. Of course, the payoff for longtime investors who kept their faith in Greenberg through thick and thin has been astounding.

But the scandals of the last several years have obviously taken a toll, and AIG's board, which has been as subservient as any one would find on Wall Street, began to ask questions. AIG is under investigation by the SEC, as well as state investigators in New York, over a deal between AIG and Berkshire Hathaway (NYSE:BRKa) (NYSE:BRKb) subsidiary General Re -- a transaction regulators believe was designed to manipulate AIG's earnings.

This investigation comes on the heels of a bid-rigging scandal that involved AIG, Marsh & McLennan (NYSE:MMC), Ace (NYSE:ACE), and others. AIG's trouble with regulators began in earnest with an investigation into its finite insurance deal with cellular phone distributor Brightpoint (NASDAQ:CELL), which allowed the company to wipe out a large quarterly loss. AIG settled this inquiry by paying a fine of $126 million.

While the board has not officially commented on its concerns, it is thought that it has grown weary of Greenberg's imperious style, which, while delivering phenomenal success for the company, has also come back to haunt it. One of the reasons the SEC gave for the size of the fine against AIG in the Brightpoint case was the supreme lack of cooperation it provided investigators. AIG has come under similar criticism in recent years regarding other incidents.

Martin Sullivan, a chief operating officer at the company, is slated to become AIG's new CEO -- only the third since the company's founding. As we noted in 2000, while AIG's persona is deeply linked to Greenberg, its pool of management talent has long been considered extremely deep.

Sullivan will have some challenges on his hands -- not only the investigations but also an insurance market in which pricing has grown much more competitive over the past two years. And finally, it is expected that Greenberg will remain as chairman, and he also will remain the company's dominant shareholder. Regardless of whether he sits in the CEO's chair at AIG, Hank Greenberg's shadow over the company -- for better or for worse -- remains extremely long.

Bill Mann owns shares of Berkshire Hathaway. The Fool has a disclosure policy.