Piedmont and WellStar, two of Atlanta's largest hospital systems, said Monday they are forming an insurance company—a bold move in a rapidly changing health care marketplace. By elbowing their way into the domain of Blue Cross Blue Shield, Aetna and United Healthcare, the hospital systems could dramatically increase the competition in the state's insurance marketplace. The move could also change the way patients get care, because the hospital systems will have a financial stake in keeping patients healthy and controlling costs.