If you decline the rental companies own insurance they will normally require a deposit just in case you damage the car. It does not matter what insurance you have, or indeed if you have any cover. The only thing that the rental company is concerned with is that you have declined their insurance. If you do take their insurance (or that offered by the travel company that you booked the car hire with) you are not required to leave a deposit for the car.
The deposit is taken by way of a credit card imprint for $1,500. Although this is not processed by your credit card company as a payment, it does temporarily reduce the available credit on your card. If you are concerned about exceeding your credit limit you could ask your card issuer to give you a short-term extension to your credit limit. I used to do this with my AmEx card until the mad fools iraised the limit to a ludicrous amount so it is no longer necessary to ask for an temporary increase.
Not all companies seem to require a deposit like this. Hertz have not requied a deposit the last 2 or 3 times I've hired with them. Dollar have always required a deposit.
Regards
Rob