Excuse me, but it is the "pre-detailed" part which I don't get it. Are you looking for library of details and drawings available from various states/county internet sites?
As far as I can remember, they are ArcInfo Shape files. If you are using AutoCad Map, you can import those files into AutoCad Map, and save them again as .dwg.
Even if you are not using AutoCad Map, I think you can find a freeware conversion program somewhere to convert them. I'll see whether I have such a conversion program on my harddisk.