The Fed also began last year to gradually reduce the portfolio of Treasuries and mortgage bonds it acquired after the 2008 crisis, reducing the amount of revenue the Fed gets from interest payments on those securities. Those holdings are the source of most of the Fed’s revenues.

The Fed’s contribution to the government’s coffers still remains well above pre-crisis levels.

The Fed made an average annual contribution to the Treasury Department of $23 billion during the five years preceding the crisis. Since 2010, the average contribution has been $86 billion.

The Fed’s bond holdings comprise federal debt and securities issued by the government-owned mortgage finance companies Fannie Mae and Freddie Mac. By diving into the marketplace, the Fed created more competition, forcing investors to accept lower interest rates — and thus reducing the borrowing costs paid by businesses and consumers, as well as the federal government.

The interest that the Fed collects on its investments is paid by the federal government, and then returned to the government. But this wash cycle still saves money because those interest payments would otherwise be made to the outside investors who would have purchased the bonds.

“It’s interest that the Treasury didn’t have to pay to the Chinese,” Ben S. Bernanke, then the Fed’s chairman, told Congress back in 2011, when the annual windfalls were still new and surprising.