Warren Buffett's Berkshire Hathaway has revealed it has been buying shares of insurer Chubb over the past few quarters, making it one of Berkshire's largest holdings. Berkshire has acquired nearly 26 million shares of Chubb for a total stake worth $6.7 billion, making it Berkshire's 9th largest holding. Insurance is a core business for Berkshire, which owns companies like Geico and General Re and recently acquired Alleghany. Chubb's stock price jumped nearly 7% in after-hours trading following the news of Berkshire's sizable stake in the company.