The collateral isn't limited to cash.

Eligible collateral for initial margin includes cash, debt securities that are issued or guaranteed by the U.S. Department of Treasury or by another U.S. government agency, the Bank for International Settlements, the International Monetary Fund, the European Central Bank, multilateral development banks, certain U.S. Government-sponsored enterprises’ (‘‘GSEs’’) debt securities, certain foreign government debt securities, certain corporate debt securities, certain listed equities, shares in certain pooled investment vehicles, and gold.

From here: https://www.gpo.gov/fdsys/pkg/FR-2015-11-30/pdf/2015-28671.p...