Chicago is an international hub for finance, culture, commerce, industry, education, technology, telecommunications, and transportation. It is the site of the creation of the first standardized futures contracts, issued by the Chicago Board of Trade, which today is part of the largest and most diverse derivatives market in the world, generating 20% of all volume in commodities and financial futures alone. Depending on the particular year, the city's O'Hare International Airport is routinely ranked as the world's fifth or sixth busiest airport according to tracked data by the Airports Council International. The region also has the largest number of federal highways and is the nation's railroad hub. The Chicago area has one of the highest gross domestic products (GDP) in the world, generating $689 billion in 2018. In addition, the city has one of the world's most diversified and balanced economies, with no single industry employing more than 14% of the workforce. Chicago is home to several Fortune 500 companies, including Allstate, Boeing, Caterpillar, Exelon, Kraft Heinz, McDonald's, Mondelez International, Sears, United Airlines Holdings, US Foods, and Walgreens.