UnitedHealth Group combines the largest health insurance operation in the United States with Optum, a healthcare services business that has become the company's growth engine. UnitedHealthcare covers tens of millions of members across employer, individual, Medicare and Medicaid plans. Optum runs clinics and surgery centers, manages pharmacy benefits and sells data and technology services across the industry. The two halves feed each other, giving the group unusual reach into how American healthcare is paid for and delivered.