So the broker interactive broker is not transferring money from futures account to stock account in excess cash. Now the reason some traders open an account futures with interactive brokers is the percieved fact that their cash is protected from broker fraud or firm insolvency. The f act that some traders even suggest that futures be 'insured' is ignorance othe risk of futures trading and howthe exchange must honor debts or losses of firms The fact that firms like mf global and interactive or any other futures FCM can trade with clients money at risk is why futures accounts are not insured. Whereas stocks are insured. In stocks the leverage is maximum 50% or no leverage at all. And co-mingling in stocks.. Stocks were not insured til 1970 and sipc didn't exist til 1970 because the risk is too high for anyone to insure it. For hedge funds who have millions sipc is useless to them. The solution isn't insurance because no major bank has become insolvent and no major brokerage has become insolvent. The people who are paying the insurance is the brokers themselves. and the fact they are paying sipc fees to their competitor like interactive broker who risk client funds in futures trading is nonsense. The fact that securities regulators approved interactive broker to sweep accounts is failure of secruities regulators. The rules are clear on co-mingling. SIPC is mostly to insure fraud by management of the broker/dealer not insure trading risk since client money is supposed to be segregated and no co-mingled with broker/dealer funds.