Chubb Ltd (NYSE:CB) is 7.5% higher in premarket trading, after Warren Buffett's Berkshire Hathaway finally revealed the Swiss-based insurer as its "mystery stock." Berkshire disclosed a 6% stake in the company, and now holds 25.9 million shares of Chubb stock worth roughly $6.7 billion, becoming its ninth biggest holding as of the end of March. Berkshire obtained permission from the U.S. Securities and Exchange Commission (SEC) to keep the purchases confidential in the third and fourth quarter of last year. ...