Idaho Code § 21-802: Division into regions.

For the purpose of this act, the state of Idaho is divided into five (5) air regions, consisting of the following counties, to-wit: the northern region shall consist of Benewah, Bonner, Boundary, Kootenai and Shoshone counties; the north central region shall consist of Clearwater, Idaho, Latah, Lewis and Nez Perce counties; the southwestern region shall consist of Ada, Adams, Boise, Canyon, Elmore, Gem, Owyhee, Payette, Valley and Washington counties; the south central region shall consist of Blaine, Camas, Cassia, Gooding, Jerome, Lincoln, Minidoka and Twin Falls counties; and the eastern region shall consist of Bannock, Bear Lake, Bingham, Bonneville, Butte, Caribou, Clark, Custer, Franklin, Fremont, Jefferson, Lemhi, Madison, Oneida, Power and Teton counties.