The Federal Reserve is moving quickly to fill in the details of how it will wind down its securities holdings in the years ahead, a process that could start this year and become the next big challenge for investors grown accustomed to easy money from the world’s most important central bank.

The Fed wants to move toward a smaller portfolio for several reasons. The economy is on stronger footing, leaving less need for support from a large bond portfolio. The large holdings have become a political liability, unpopular in Congress. Moreover, getting started now could relieve pressure on possible new leadership in 2018, when Fed Chairwoman Janet Yellen’s term ends. Finally, officials want room to ramp it back up in a crisis if needed.

The Fed moved closer to agreement on the outlines of a plan at their March 14-15 meeting, according to an official account released this month and interviews with officials.

Officials want their benchmark federal-funds rate to remain the primary tool for managing monetary policy, which means once they start shrinking the balance sheet, they want that process to run in the background with little ripple in markets.

If the economy performs in line with the Fed’s forecasts, the bank could begin allowing its holdings of Treasury securities and mortgage bonds to mature without reinvesting all of the proceeds into new securities later this year or early in 2018.