Your butterfly is probably from the family Papilionidae and the subfamily Papilioninae; also known as the Swallowtails.  They get their name because of the have on (or two) long, tail-like extensions off the rear of their hindwing.  They are large butterflies that are often black and yellow in color.  Below are pictures of all of the Swallowtails that are known to occur in Idaho.

Does your butterfly look like this?