Shares of U.S. health insurer UnitedHealth dropped Friday following news that the Justice Department has joined a lawsuit filed by a former executive. The lawsuit accuses the company of overcharging Medicare by hundreds of millions of dollars. UnitedHealth stock, a component of the Dow Jones industrial average, ended the session about 3.7 percent lower, closing at $157.62 a share. Shares of several other insurers also fell. They include Aetna, Humana,and WellCare, which whistle blower Benjamin P...