Warren Buffett gained more wealth than any other U.S. billionaire, adding $37 million a day, according to one study.
Through Dec. 11, Buffett's net worth—at least on paper—shot up by $12.7 billion to $59.1 billion in 2013 from $46.4 billion at the start of the year, according to Wealth-X, the wealth research firm. That works out to a paper gain of $1.5 million an hour.