AIRLINES

Thirty years after a troubled investment in USAir, Buffett no longer views the airline industry as suicidally competitive on price, after several carriers righted themselves through the bankruptcy process and competition shrank. Buffett's longtime second-in-command, Vice Chairman Charlie Munger, has also had a change of heart, comparing today's airline industry to the early days of the railroad. "It was a terrible business for about 80 years," Munger said. "But finally they got down to four big railroads and it was a better business. And something similar is happening in the airline business." Berkshire currently owns close to 10 percent of American Airlines, Delta Air Lines and Southwest Airlines, and more than 8 percent of another carrier, United Continental.

INSURANCE

While commanding a smaller percentage of the Berkshire universe than it once did, insurance still typically generates more than 20 percent of Berkshire's operating earnings. Its major businesses include Geico, now the second-largest U.S. automobile insurer, and Berkshire Hathaway Reinsurance Group, which covers large and unusual risks and is a major source of Berkshires roughly $123 billion of "float," or premiums collected in advance that can fund Berkshire's growth elsewhere. "Berkshire's unrivaled financial strength allows us far more flexibility in investing our float than that generally available to P/C companies," Buffett said in this year's annual shareholder letter, referring to property and casualty insurers. "When other insurers are constrained, our choices expand."

APPLE

Berkshire began investing in Apple in 2016, starting with a $1 billion position taken not by Buffett but by his investment managers Todd Combs and Ted Weschler. Buffett began investing in Apple later, and it has since grown into Berkshire's largest single common stock holding, worth more than $50 billion. Buffett believes in 'economic moats,' a term he popularized to describe one company's competitive advantage over all others. And Buffett thinks the iPhone maker has a deep moat. "I went into Apple because I came to certain conclusions about both the intelligence with the capital they deploy, but more important the value of their ecosystem and how permanent that ecosystem could be," he said at Berkshire's annual meeting last year.

MANUFACTURING, SERVICE and RETAIL

These more economically sensitive lines of business generate more than half of Berkshire's revenue — the low-margin McLane food distributor generates about 20 percent by itself — from a disparate group of companies including Benjamin Moore paint, Brooks running shoes, Clayton Homes manufactured housing, Dairy Queen ice cream, Duracell batteries, Fruit of the Loom underwear, Johns Manville insulation, Lubrizol chemicals, the NetJets fractional plane business, dozens of newspapers including the hometown Omaha World-Herald, Precision Castparts industrial parts, See's Candies, and Scott Fetzer, home to Ginsu knives and the company once known for the World Book encyclopedia. "Investors who evaluate Berkshire sometimes obsess on the details of our many and diverse businesses, our economic 'trees,' so to speak," Buffett said in this year's shareholder letter. "A few of our trees are diseased and unlikely to be around a decade from now. Many others, though, are destined to grow in size and beauty."

RAILWAYS

Back in 2009, less than a year removed from the Great Recession, Buffett turned heads by deciding to buy the Burlington Northern Santa Fe railroad, paying $26.5 billion for the 77.5 percent that Berkshire did not already own. At the time he described the deal as an "all-in wager on the economic future of the United States". BNSF operates one of the largest freight railroad networks in the country.

BANKS

Berkshire owns multibillion-dollar stakes in six of the largest U.S. banks, including nearly 10 percent stakes in both Bank of America and Wells Fargo, and owns more than one-sixth of American Express. Buffett picked up some of these positions during the depths of the 2008 financial crisis at less than a third of their current value, and first purchased shares of American Express during a banking panic in the 1960s. "Only when the tide goes out do you discover who's been swimming naked," he once said.

ENERGY

Berkshire Hathaway Energy generates about 10 percent of Berkshire's overall profit, mainly from power transmission businesses, such as electric and gas utilities, in the midwestern and western United States, the Canadian Rockies and England. The company also includes HomeServices of America, whose Berkshire Hathaway HomeServices unit has become the largest U.S. residential real estate brokerage by number of units sold, and which is expanding into several non-U.S. countries, including Dubai last month.