UnitedHealth Group shares fell more than 15% in early morning trading on Thursday, after The Wall Street Journal reported that the U.S. Department of Justice was investigating the company for potential Medicare fraud. The latest news adds to the health insurer's litany of woes, including multiple government inquiries, a sudden change in top leadership and a pulled outlook in the face of soaring medical costs.Investors and analysts noted that while details on the investigation were limited, they did heighten ...