The economy appears healthy. Economic growth is 3.5 percent, nearly reaching the 4 percent growth President Trump promised when campaigning. Unemployment is 3.7 percent. Yet one number points out the fragility of the others: $66.5 billion.

That’s how much the Federal Reserve, the U.S. central bank, reported recently in “unrealized losses” on its investment portfolio. These losses are the delayed bill for the Fed’s 2008 bailouts of the housing market — and a reminder that the economy is still dependent on extraordinary government support.