You may want take your alternator to and have it checked out. The regulator will make adjustments based on the cars electrical output needs. So to see lower voltage especially at idle is not abnormal. If you don't feel like taking it out and having it reviewed by Autozone. Start the car, check the voltage. Turn on all accessories including the lights and then check the voltage. You should see the voltage increase as the system tries to supply the correct amperage to all the accessories.
Additionally if you don't see what you want to see in voltage increase, look to see if the battery light comes on when you turn the car to "on". If you don't get this from your dash then I would start by replacing this bulb.
Just my .02 cents!