What they said, pretty normal for a big drop at idle.
As mentioned check the stator wires, that connector has been know to get hot enough to melt. if it did just eliminate it and solder the connections together. Also check the battery connections, and ground connections, clean them even if they look clean already.
Using a digital voltmeter check the voltage across the battery. should be 12.6 volts with the bike turned off, and 14 volts over 2000 rpm. The voltage will be less idling with everything turned on, the alternator just is not spinning fast enough to meet the demand at idle.