Quote from Bolimomo:
I don't know about IB. But many brokers have automatic swap set up for margin accounts. This means if you don't use your money to buy stocks and hold overnight, your money will be in a money market account earning interest for you. (Though not much these days). If you use your money to buy stocks, it swaps out enough of it to pay for the purchase.
e.g. You have $50k in your account. If you haven't purchased any stocks, your $50k will earn you some interest in the money market account.
If you buy $30k worth of stock1, the broker will automatically swap the $30k out of your money market account. You don't pay magin interest because you did not borrow any money to buy stock1. But you only have $20k left to earn you some money market interest.
Let's say you further use another $30k to BUY stock2. Then you are borrowing $10k from the broker. You don't earn any money market interest. And you will be paying margin interest (at a much higher rate) on the $10k borrowed. You don't pay margin interest on all $60k eventhough you have a 60k position.
This is how I understand it from my past tradings.
What I am not sure is when you use money to short stocks. Because shorting is done in margin accounts. If you have $50k and shorted and held $30k of stock1, will you be paying margin interest on all $30k. You could very well be.
You need to re-read your brokerage agreement. It makes it very clear in your contract. BTW, one of the reasons why every trade is marginable is it gives the broker the right to lend out your stock to someone else. If you paid cash for your stock they would not be able to lend it out.
Yes, if your cash is sitting idly in your account it will earn interest. If you have 50k in your account and you buy 30k in stock, you will pay margin interest on 15k and you will earn interest on the remaining 35k in your account.