White smoke out of exhaust pipe

I just bought a 1998 Dodge Caravan with 67,000 miles on it. I noticed that it has white smoke coming out of it when it idles. Once on the rode and driving there is not any smoke unless I stop and it idles again. I called the previous owner and she said it has always done that the entire 7 years she has owned it. They had it regularly maintainced and no problems were noted. I was wondering what it could be that wouldn't have caused any major problems up to this point.