Read all about it! A historic event is finally happening. First came the end of the era of quantitative easing; then the resumption of interest rate hikes; now - finally - the US Federal Reserve will begin to reduce the $4.5 trillion balance sheet it acquired since the 2008 financial crisis. The majority of the Fed's balance sheet consists of Treasuries and mortgage-backed securities accrued during quantitative easing. The Fed plans a very gradual draw-down, rising slowly to a final drawdown of $50 billion / month, or roughly in-line with analyst's expectations.
https://www.bloomberg.com/news/articles/2017-09-20/fed-asset-shrinking-to-start-next-month-rate-hike-seen-in-17
