Securities are held in custody by Electronic Transaction Clearing, Inc. ("ETC") and USD cash balances are held in money market accounts at Dreyfus, a wholly owned subsidiary of the Bank of New York Mellon.  All your holdings and cash are registered in your name in a segregated account and are insured for up to $US500,000 under SIPC insurance.

This is slightly different to the Australian system where each account has its own Holder Identification Number with the exchange clearing system (CHESS). In overseas markets, the custody arrangement is far more common.