Exactly what are you trying to do? A 2D skybox texture should stay a world brush, it should never be tied to an entity. A func_detail tells VVIS to disregard the brush you've func_detail'd when calculating vis info. If it's connected to the void, it will create a leak.
I ended up having to re-draw the skybox, but it didn't fix it at all.
If your skybox around your map is func_detailed, that is definitely a problem, though.