IB and shorting a stock

I've a pretty basic question when it comes to short selling.

I've a portfolio margin account with IB. So based on that my buying power gets reduced by 15% of the overall equity value when buying or short selling a stock.

What I don't know is if there are any additional cost, when holding positions overnight.

e.g. Buy 100 SPY, Sell 100 QQQQ
(~ 100×133.11×0.15 = 1996 margin for the SPYs + 100×58.44×0.15 = 876 in margin for the QQQQs, total of 2872 in margin).

I read in other threads that other brokerages charge additional money (e.g. 10% interrest on the short position), so I pinged the customer service folks @ Interactive Brokers and they sent me to a webpage that didn't really help me out a lot.

So my question is, (commission aside), are there any additional fees/interest charges on the position mentioned above or does this not apply based on the portfolio margin agreement?

Thanks for any input