Berkshire Hathaway Inc., led by Warren Buffett, revealed a $6.7 billion stake in insurance company Chubb Ltd., ending months of speculation about its mysterious position in a financial company previously concealed in regulatory filings.

Berkshire disclosed the stake in a Wednesday filing, reflecting its positions at the end of the first quarter. Some refer to the headquarters of Buffett’s holding company in Omaha, Nebraska, as the "Oracle of Omaha."

According to Bloomberg, Berkshire began building this stake in 2023, but it hadn't been reported earlier because the Securities and Exchange Commission allowed Berkshire to keep it confidential. However, separate quarterly filings indicated Berkshire’s holdings in banks, insurance, and financial companies were increasing, while the company was retreating from other sectors, including consumer products.

"Millions of people watch what Buffett does," said David Kass, a finance professor at the Robert H. Smith School of Business at the University of Maryland, explaining why Berkshire prefers confidentiality while accumulating large positions. "Warren Buffett would be more sensitive about this than others."

Chubb's stock surged in after-hours trading, gaining up to 9.9%.

Buffett’s Berkshire is deeply familiar with the insurance industry, owning a range of companies such as Geico and National Indemnity. The billionaire investor has called Berkshire’s property-casualty insurance business the "core" of the group, contributing to the creation of "float" that can be reinvested.

The group has also invested in other insurance sector businesses. Berkshire holds a stake in Aon Plc, a major broker, and has previously bet on competitors like Marsh & McLennan Cos.

Chubb is one of the largest property and casualty insurance companies in the U.S. and operates in 54 countries worldwide. Its CEO, Evan Greenberg, is the son of Maurice "Hank" Greenberg, who led American International Group Inc. for many years. Evan Greenberg built Chubb through the 2016 merger of Ace Ltd. and Chubb Corp., creating a giant insurer that covers a range of risks, including cyberattacks and maritime issues.

Chubb insured the Francis Scott Key Bridge in Baltimore, which collapsed when a cargo ship struck it in late March. It is reportedly set to pay $350 million to the state of Maryland.

Buffett already revealed some recent changes in Berkshire’s holdings at the company’s annual meeting in Omaha earlier this month. He reduced his stake in Apple Inc. to $135.4 billion at the end of the first quarter, as the iPhone maker faces several challenges, including an antitrust fine, declining sales in China, and a failed car project.

The billionaire investor praised the tech giant at the meeting—attended by Apple CEO Tim Cook—and stated that Apple would remain Berkshire’s largest investment unless there are dramatic changes.

Berkshire's cash pile reached a record $189 billion at the end of March. Buffett said at the annual meeting that it is "a fair assumption" that it will reach $200 billion by the end of the current quarter.

Funds with more than $100 million in assets must disclose their holdings within 45 days of the end of each quarter, providing a glimpse into the positions of secretive fund managers, including hedge funds and large family offices.