“We believe it is incredibly important, in the business we’re in, that we insure all Americans,” Chief Executive Mark Bertolini said. Bertolini’s comments contrast with a warning in November from UnitedHealth Group Inc, the largest U.S. health insurer, that it was considering exiting the exchanges in 2017 due to weak enrollment and high costs. Aetna’s exchange business represents 5 percent of its total membership and 6 percent of its total operating revenue, Bertolini said in remarks at the J.P. Morgan Healthcare Conference in San Francisco.