Suspect voltage regulator as previously pointed out. 2 screws come out of regulator which is on back of alt. Easily accessible if you put front of car up on ramps first - pull large plastic encapsulation shroud underneath.
Mine was reading about 13.20 at idle, then one day out of the blue the idle fell off and the SRS/ABS light came on. This last situation is indicative of the brushes in the volt. reg. finally giving up.
New brushes are about 1/2" long. Pull your present ones and see how long they are. This is a VERY easy job. Takes longer to setup for it than it does to remove/install new voltage regulator(brushes).
After changing brushes, I see about 13.95 at idle first thing in the morning with no heavy current users active. After the car is fully warmed up, I see it around 13.60 - 13.70.