Many of the largest companies are also household names — but not all. While often the largest public companies are recognizable, some of the largest private companies are far less known. For example, privately owned Cargill, a large conglomerate heavily involved in agricultural and food production, had revenues of nearly $135 billion in 2014. This was more than the 2014 revenue of widely known and public AT&T of about $132.4 billion.

Based on data from PrivCo, 24/7 Wall St. reviewed the 20 private companies with the largest revenues. All of these companies had revenues of at least $20 billion, with Koch Industries and Cargill Incorporated reporting revenues well over $100 billion, the two highest. The companies employ between 5,000 and 200,000 workers each, with Ernst & Young employing the most workers at roughly 200,000.

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These companies do not have much in common beyond being privately held and often also being quite old. Several of the companies were founded in the 1800s or early 1900s. Cargill, the largest private company, is also one of the nation’s oldest, founded in 1865 at the end of the Civil War. The size of these companies is often the result of success compounded over decades, leading them to become dominant players in their respective industries.

In an interview with 24/7 Wall St., Radek Jagielski, Senior Analyst at PrivCo, explained how large capital bases accumulated over many years are a major advantage for these companies. With so many resources available for investment, “there’s really no reason for them to go public and dilute their ownership, ” said Jagielski. In addition, Jagielski explained, large capital bases can be used to “acquire competitors before they become real threats.”

Many of the oldest companies on the list are family owned businesses, with namesakes still held by current owners. In several cases, family members have retained an active role in the management of the company. For example, Bechtel, one of the world’s largest construction and engineering contractors, is chaired by Riley Bechtel, while Brendan Bechtel is the company’s president and COO.

In all cases, the families’ retention of control of these large companies has resulted in extraordinary inherited wealth. For example, the Koch brothers, who control Koch Industries, are estimated to be worth more than $40 billion each.

Many of the largest private companies are mutual insurance companies. These are insurance companies that are set up to be owned by their policyholders, and so are inherently private companies. These companies, such as State Farm Mutual and Health Care Service Corporation are among the largest insurance companies in the nation.

Policyholders at these insurance companies have voting rights and do not have to worry about being taken over. According to Jagielski, this means these companies can make more long-term strategic plans. In addition, he said, policyholders are able to make changes when they’re unhappy with the insurance product. Less access to capital is one disadvantage to this model. However, as all of the insurance companies on this list have been around for nearly 100 years or more, access to capital is not a major issue.

To identify the largest private companies, 24/7 Wall St. reviewed the 100 U.S. private companies with the largest annual revenue figures from PrivCo. Revenues are as of the most recent period available. Company headcounts, headquarter locations, and the industries in which the companies operate also come from PrivCo. Financial data and company histories come from company websites.

These are 20 largest private companies.