Moody’s, which has rated the U.S. Aaa since 1917, said in the report that the dollar’s role as the world’s main reserve currency means “the U.S. government can support higher debt levels than other governments.” The greenback represents 60.7 percent of the world’s currency reserves, compared with 26.6 percent for the euro, which has the next biggest portion, according to the International Monetary Fund in Washington