Interactive Brokers Group Inc. (Interactive Brokers (IBKR)) reported its second-quarter results after the market closed on Tuesday, and the numbers were better than Wall Street expected. The electronic brokerage firm posted revenue of roughly $1.90 billion, beating the analyst consensus of $1.75 billion. Adjusted earnings came in at $0.69 per share, also ahead of the $0.64 per share estimate.

So what drove the beat? A few things. Commission revenue was up 30% year-over-year, thanks to higher customer trading volumes. Stock trading volume rose 14%, and options trading volume climbed 17% compared to the same quarter last year. That's a lot of clicks and trades.

Net interest income — the money the firm earns on the difference between what it pays on customer deposits and what it earns on loans and securities — increased 23% to $1.06 billion. The main drivers were higher average customer margin loans and larger customer credit balances. Basically, more people are borrowing to trade, and more cash is sitting in accounts earning interest for the firm.

Other fees and services revenue jumped 40% to $87 million, led by payment for order flow and risk exposure fees. Execution, clearing, and distribution fees rose 22% to $142 million.

The customer base is growing fast, too. Interactive Brokers added accounts at a 34% clip year-over-year, ending the quarter with 5.19 million customer accounts. Customer equity hit $930.30 billion, up 40% from a year ago. That's a lot of assets under custody.

The board also declared a dividend of $0.0875 per share, payable on Sept. 14 to shareholders of record as of Sept. 1. Not a huge payout, but a steady one.

Management will discuss the quarter in more detail on an earnings call with investors and analysts at 4:30 p.m. ET.

Shares of Interactive Brokers were up 1.57% in after-hours trading Tuesday, changing hands at $95.90 at the time of publication.