Your butterfly is probably from the family Pieridae and the subfamily Pierinae; which include the Whites, Marbles, and Orangetips.  On the tops of their wings they are predominately white with black patterning on the wings.  Veins on the underside of the hind wings appear green.  Below are the species that are known to occur in Idaho for which we currently have images (Margined White is the only species for which we are missing an image).

Does your butterfly look like this?