Warren Buffett's birthday this week was good news for Berkshire Hathaway investors who could celebrate another year of the Oracle of Omaha's leadership. But the milestone also reminded shareholders they need to think about who will run the evolving company after Buffett is gone.

The more than 80 businesses that Buffett has assembled at Berkshire are mostly humming along well, posting a profit of more than $3 billion in the most-recent quarter. Its Class A stock closed at $127,183 on Thursday — less than $3 away from its 52-week high set in early August.

So there's plenty for shareholders to celebrate.

Editor's Note: Unthinkable Haunts Investors: Evidence for Imminent 90% Stock Market Drop. 

But Buffett's advancing age — he turned 82 on Thursday — coupled with his radiation treatment for prostate cancer this summer, keeps his mortality on the minds of many Berkshire investors. Buffett has said the cancer isn't life-threatening, and he is feeling good.

Buffett has outlined Berkshire's succession plan and reassured investors that his board has chosen a successor and two backup candidates, although he won't say who the company's next CEO will be. Buffett is both chairman and CEO.

Buffett has no plans to step down, saying he enjoys the dealmaking too much, even though he hasn't landed a sizeable acquisition since buying chemical maker Lubrizol for $9 billion last year.

Whoever takes over Berkshire after Buffett will inherit a sprawling conglomerate that is evolving with each new acquisition. The Omaha-based company now relies less on the insurance companies and investments it has long been known for and more on its railroad, utility and manufacturing companies.

"The insurance is increasingly more of an engine that runs in the background instead of the driver of the business," said Jeff Matthews, an investor who wrote "Secrets in Plain Sight: Business & Investing Secrets of Warren Buffett."

Buffett did not respond to an interview request for this story.

Berkshire Hathaway Inc.'s second-quarter earnings report earlier this month showed that easily more than half of the company's profits come from mostly mundane companies like its electric utility MidAmerican Energy, Lubrizol, BNSF railroad, manufacturing and tool companies.

The trend has been strengthening ever since Buffett's firm acquired the Burlington Northern Santa Fe railroad in 2010, but it's even more apparent now that Lubrizol has been part of Berkshire for nearly a year.

"Even in the best of operating environments, the insurance side of the business will never out-earn the non-insurance side ever again," said David Rolfe, chief investment officer at Wedgewood Partners, which counts Berkshire as its second-biggest investment.

The shift in the mix of Berkshire's businesses could make the company more attractive to investors who found the world of insurance and reinsurance complicated. But analysts say the nature of the company isn't likely to change much, even though Berkshire's profits may be coming from different places.

"He's got diversity," said stockbroker-author Andy Kilpatrick. "Rather than just a stock company or an insurance company, it's an operating company."

Kilpatrick says he doesn't really think the nature of the company has changed because Berkshire just tacks acquisitions on to the existing structure with little attempt to integrate them.

"It's not a simple company. It's a complicated company, but it's a successful company," said Kilpatrick, who wrote "Of Permanent Value: The Story of Warren Buffett."

Berkshire's insurance and reinsurance companies such as Geico had long driven the company's profits. And they generate $71.1 billion in "float" that Berkshire is able to invest between the time when customers buy policies and when claims are filed.

Regardless of how big Berkshire's insurance companies are, they will remain important to the company because of the risk involved with some of the big policies written by the company's reinsurance division.

For example, in one 2006 deal Berkshire agreed to cover up to $13.9 billion in potential asbestos claims in exchange for $7.12 billion. And the derivative contracts Buffett wrote insuring the level of certain stock market indexes could create multibillion-dollar losses if they weren't priced right, although Buffett has said he believes those deals will prove profitable.

Matthews said insurance is clearly still the riskiest part of Berkshire's business, so Buffett, and whoever follows him, must understand it.

"If you do a bad job at the railroad, your earnings might get hurt, but the railroad is not going away. If you do a bad job in insurance, your whole company can go away," Matthews said.

Berkshire plans to split Buffett's job into three parts once he is gone. The next CEO will run Berkshire, but two other men hired by Buffett in recent years will oversee investment. Buffett wants his eldest son to succeed him as chairman.

As a result of the shift in Berkshire's business mix, the company's earnings should become less volatile over time, and it will increasingly be driven by the results of its operating companies, like Iscar tools and Acme brick.

But who knows how the mix of companies might change if Buffett finds a way to use the roughly $41 billion cash Berkshire has on hand for another big acquisition.

"At any moment, he could buy some other insurance thing and that could look big again," Kilpatrick said.

Besides insurance and manufacturing, Berkshire's subsidiaries include clothing, furniture, ice cream, private jet and jewelry companies. It also has major investments in such companies as Coca-Cola Co., IBM and Wells Fargo & Co.

Editor's Note: Unthinkable Haunts Investors: Evidence for Imminent 90% Stock Market Drop.