e system idle process is what is refered to as a sentinal process. It basically gives the CPU something to do when it does not have any other processes scheduled (this allows nicer code in the scheduler, since it does not have to have special cases when there are no runable/ready processes). Also, this process can be useful from an OS cleanup point of view (looks for deadlocks, etc.)
It runs at the lowest priority, and so will get cycles when nothing else is requesting CPU attention. So if you are not doing anything, then it is perfectly normal for it to have high CPU usage.