Louisville-based Humana said CEO Mike McCallister plans to retire in the next year to 18 months. The company named a healthcare executive from outside the company to be his replacement. Bruce D. Broussard, CEO of McKesson Specialty Health, a $9 billion subsidiary of San Francisco-based McKesson, will join Humana as president. Broussard, 49, will take over when McCallister steps down, in December, Humana said. McCallister, 59, has led the health insurance company for more than a decade.