In a historic transition for the large regional insurer, Health Net officials have an-nounced plans to sell to Centene Corp., a national provider of government-based insurance plans. Aside from merger negotiations, Health Net focuses on growing its Medicaid expansion population in Arizona and California, where it also offers lower premium products on the state insurance exchanges. Years of declining commercial enrollments have encouraged Health Net to focus on Medicaid and high-acuity members dually eligible for Medicare and Medicaid.