Medicaid coverage provider Centene will pay about $6.3 billion to buy fellow insurer Health Net, as managed-care companies look to bulk up while adjusting to the federal health care overhaul. The deal announced Thursday gives St. Lous-based Centene a chance to grow in two hot areas for health insurers, the state- and federally funded Medicaid program for the poor and people with disabilities, and the federally funded Medicare Advantage program, which has seen its overall enrollment triple over the past decade. The health care overhaul is expanding Medicaid coverage to millions as it seeks to provide health insurance for more people. Centene said the acquisition of Health Net would strengthen its presence in the nation's largest Medicaid market, California, and would create a combined company with about 6 million Medicaid members, making it one of the largest in the country.