The nation's largest insurer said it booked $720 million in losses last year by offering plans under Obamacare, and warned Tuesday that it might still withdraw altogether from the health law by next year. UnitedHealth Group told investors that it expects more losses due to Obamacare in 2016, countering an otherwise upbeat earnings report for the company, and serving as a challenge to President Obama, who wants to leave the law on firmer footing. The administration brushed off the news, saying other major insurers are sticking by the exchanges and that it is focused on the final two weeks of...