I would do some checking and troubleshooting. Sounds like the alternator may be charging wide open, the only thing holding the voltage down to a reasonable level are the batteries. If you run it like this very much, you may have over heating wiring or over heating batteries as a result.
1st check the "A" terminal on the regulator. It should have a yellow wire going to it, and it's pretty short, should be running to a spot were it can get battery + voltage. The "A" terminal is where the regulator samples the system voltage, so it can give the right charging info to the alternator. If this wire has a poor connection or not connected at all, the regulator thinks the system voltage is low, and will throw the alternator wide open to try and bring the voltage up.
Check the "F" wire on the regulator. This wire is how the regulator controls the alternator output. This goes directly to the field of the alternator. More voltage on this wire means more charging from the alternator. Make sure it goes to the proper terminal of the alternator. Sometimes it's wrapped in the output wire from the alternator, which is hooked directly to the battery +. If one of these wires gets hot, it can melt the two together, putting a full 12v on the "F" wire. This would throw the alternator to wide open.
Make sure you never accidentally short the "F" wire to ground, it is a sure way to burn the regulator out.
If you are getting 18v at idle, your alternator is definitely good. You just have a control problem with it.
