Four iconic American CEOs are making way for a new generation of business leaders . Warren Buffett , Bob Iger , and Doug McMillon have stepped down from the top jobs at Berkshire Hathaway, Disney, and Walmart, respectively. Tim Cook has announced he'll resign as Apple's CEO in September. Their successors aren't household names , but they all share one common characteristic: they're detail guys who know their companies inside out. These nuts-and-bolts leaders pay attention to every little thing,