UnitedHealth Group revealed Thursday it is facing a Justice Department investigation over its Medicare billing practices, adding to a string of setbacks for a company that owns the nation's largest and most powerful private insurer.In a securities filing, the company said it has started complying with formal criminal and civil requests from the DOJ, and that it reached out to the department after reports of the probes surfaced. UnitedHealth also said it has launched a third-party review of its b...