WASHINGTON (AP) — Moody's Investors Service upgraded the outlook for U.S. government debt to 'Stable' from "Negative" and affirmed the United States' blue-chip Aaa rating.

The rating agency cited a surprising drop in the federal deficit — the difference between what the government collects in taxes and what it spends. The U.S. government is on track to report its lowest annual deficit in five years.

Through the first eight months of the budget year, the deficit has totaled $509.8 billion, according to the Treasury. That's nearly $400 billion lower than the same period last year.