I'm surprised they only execute ~1,000,000 a day. I expected a lot more. http://individuals.interactivebrokers.com/en/p.php?f=ibgStrength
IB customer cash is held to a large extent in deposits with multiple large banks, U.S. Treasury bills, FDIC-backed corporate bonds and federally-approved AAA rated money market funds (less than 2% of customer assets) -- all subject to continual review by IB's senior management. The majority of the bank deposits are held with JP Morgan, Deutsche Bank and Bank of America, and to a lesser extent with Citibank, HSBC, Royal Bank of Canada and U.S. Bank. (Source IB)