By Sriparna Roy (Reuters) - UnitedHealth expects about 78% of its members to be enrolled in top-rated Medicare insurance plans next year, in line with its expectations, the top U.S. health insurer said in a filing on Tuesday. Shares of the company were up 3.21% at $330.52, as strong membership for plans with higher star ratings would mean bigger payments from the government to the insurer, and the bonus payments can be worth hundreds of millions or billions of dollars. The estimate of memberships in 4-s ...