Americans should be required to buy health insurance, bringing healthier people into plans that will help bring down costs, Ron Williams, chairman and CEO of Aetna, told a Detroit audience. Williams said he favors plans similar to those adopted by some states that require everyone to have health insurance: either through a workplace plan or through a plan consumers buy on their own, with help from state subsidies for those who can't afford it. A mandatory system would help bring down costs and end the problem of enrolling healthy applicants and rejecting those with prior medical problems because costs and risks would be spread over a larger group of people, Williams said.