Shares of Wesco Financial Corp. surged Thursday after Warren Buffett’s Berkshire Hathaway Inc. announced plans to buy the 19.9 percent of the Pasadena company it does not already own.

In a regulatory filing, Berkshire, which has owned its majority stake in Wesco for 30 years, said it will buy the remaining shares for about $500 million in cash and Berkshire Class B stock. Berkshire said it will offer to pay an amount equal to Wesco’s book value per share at the close of the transaction.

The offer requires approval by a majority of Wesco’s independent directors.

Wesco’s stock jumped nearly 19 percent after the filing. In afternoon trading, shares were up $39.25, or 12 percent, to $364.

Wesco – a diversified company that operates insurance, steel cutting and furniture rental subsidiaries – is run by investor Charles Munger, vice chairman of Berkshire and a longtime partner of Buffet.

In May, the 86-year-old Munger was named the 17th wealthiest person in Los Angeles by the Business Journal, with an estimated net worth of $1.9 billion.