What Grazr said.
You shouldnt have entire detail map.
Really what you want to do while mapping is start with base geometry. test it, make sure its enclosed etc. this will stay as your world geometry. the rest can be made into detail if you like. but only detail items should be. you may run into errors like Areaportals dont touch two sides, or brush does not contain solids in portal 0, at coordinate . . .
Things like trusses, beams, stairs, Roofs on outdoor buildings, or building that you cant touch. complex items like columns or spirals. anything that isnt a displacement that is detail.