Cash left uninvested in a US brokerage account rarely sits still. It is swept each business day into a separate arrangement, and the destination determines what it earns and how it is protected.
A brokerage is not a bank
Brokerages are not deposit-taking institutions, so cash held for customers has to be placed somewhere that can hold it under an appropriate framework.
Sweep programs solve this by automatically transferring idle balances into partner bank deposits or into money market funds at the end of each day.
The customer sees a single cash line in the account, while the money itself sits in whichever vehicle the sweep program uses. The balance is available for trading the next morning as though it had never moved.
The destination changes the protection
Cash swept into partner bank deposits sits under federal deposit insurance, subject to the applicable limits at each receiving institution.
Cash swept into a money market fund is a securities position instead. It carries brokerage account protection against firm failure, which is a different thing from insurance against loss of value.
Neither arrangement is inherently better, but they answer different questions, and the account documentation states which one applies.
Bank sweep programs spread balances deliberately
Because deposit insurance applies per depositor per institution, programs distribute large balances across a network of banks rather than concentrating them at one.
That expands the total insured amount well beyond a single institution's limit, which is the main appeal for larger cash holdings.
It also means the list of receiving banks matters, since balances a customer already holds directly at one of them count toward the same limit.
The yield gap is where the economics sit
In a bank sweep, the brokerage receives a payment from the partner banks and passes through a portion as the rate credited to the customer.
The difference between what the banks pay and what reaches the customer is revenue for the brokerage, and it widens or narrows with short-term rates.
Money market sweeps typically pass through more of the prevailing short-term yield, less the fund's own expenses, which is why default settings are worth reading.
Defaults are chosen by the firm, not the customer
Every brokerage designates a default sweep vehicle, and accounts use it unless the customer actively selects an available alternative.
Because the default is applied automatically, cash can sit for years in whichever option the firm nominated at account opening. Dividends, proceeds of sales and deposits all land there without any further decision.
The mechanism is invisible by design, which is precisely why the terms governing it determine the outcome for anyone who never changes the setting.