The amperage the alternator delivers at idle will vary, depending on the load presented to it. If you have your lights and A/C on, with a discharged battery, your alternator will definitely deliver a heavier charge than if all is off and charged. Your output voltage, however, should sit between 13.4 and 14.1 volts, for a healthy alternator. Anything over 14.4 volts should raise concern, for funky things may start happening above that.
If everything is perfect, you should have a rather low draw on your alternator (5-10 amps, maybe) at idle. But, like I said before, monitor your voltage...