Interactive Brokers had a record quarter — where the money actually came from

Interactive Brokers reported its second-quarter results on Monday, and they were records almost across the board. Net revenue came in at $1.90 billion, up from $1.48 billion a year earlier; diluted earnings were $0.69 a share, from $0.51; and the pretax profit margin was 77% — the seventh quarter running above 70%. For a firm that competes largely on low costs, that is an unusually profitable business.

What the numbers say about the platform

Strip out the profit line and the more useful signal is growth. IBKR ended the quarter with 5.19 million customer accounts, up 34% on the year, and total client equity — everything customers hold on the platform — rose 40% to $930.3 billion.

Management pointed to the reasons plainly: heavy trading volumes and higher interest-earning balances, helped along by a busy few weeks in markets — strong tech earnings, the semiconductor selloff that had everyone trading chips, and the SpaceX IPO. When markets are volatile, an active-trading broker does more business.

Where the record money actually came from

Two engines drove the quarter, and only one is the obvious one. Commissions rose 30% on brisk trading in stocks, options and futures — that's the part you'd expect from a broker. The other is quieter: net interest income was up 23%. Client uninvested cash at IBKR hit a record $182.4 billion, up 27% on the year.

That is cash sitting in customer accounts, not invested in anything. The broker earns interest on it. Some of that interest is passed back to the customer and some the broker keeps — and on a balance that size, the slice the broker keeps is real money, quarter after quarter. Even a "low-cost" broker can earn handsomely from you without an obvious fee — simply by holding your idle cash.

What to take from it

Nothing here is a reason to do anything with IBKR's own shares — that's not our call to make. But if you're one of those five million accounts, the practical takeaway is this: the rate you're paid on your own idle balance — and whether it keeps pace with central-bank rates — is worth knowing, and it's the kind of thing we track across brokers on our cash-interest pages.

This is a summary of what Interactive Brokers reported, not investment advice.