Buffett has the authority to initiate stock buybacks at any time he thinks the share price is below Berkshire's intrinsic value. The only caveat is that the conglomerate's cash, cash equivalents, and U.S. Treasury holdings can't fall below $30 billion. That hasn't been a problem in a while, considering Berkshire's cash position has remained above$300 billion throughout the last 12 months. Warren Buffett's favorite stock has been a huge winner for a long time. But are those days in the past? Most investors ...