Berkshire Hathaway Inc. has made a formal offer to buy the roughly 1.4 million shares it doesn't already own of subsidiary Wesco Financial Corp.

Berkshire CEO Warren Buffett said in documents filed with the Securities and Exchange Commission that a proposal was delivered to Wesco's board Wednesday.

If most of Wesco's independent board members and shareholders agree, Berkshire will acquire the remaining 19.9 percent of the Pasadena, Calif., company.

The cash and stock deal would be worth roughly $500 million, but the price could change because Berkshire offered to pay the book value of Wesco's stock at the time of closing.

Wesco is a conglomerate that owns insurance businesses and an eclectic mix of operating companies, similar to Berkshire.