A new report from UnitedHealth Group Inc. takes aim at the way U.S. doctors get paid, saying the nation could save up to $1 trillion over the next decade in health care costs if it were possible to "unleash the potential of payment reform initiatives." The report, which comes out Wednesday, analyzes the savings as well as likely pitfalls to adopting a number of different methods to move away from the current "fee-for-service" approach, which pays doctors based on the number of services they provide.