  We used the inverted resistance method to extend the bulk resistivity of
SmB$_{6}$ to a regime where the surface conduction overwhelms the bulk.
Remarkably, the bulk resistivity shows an intrinsic thermally activated
behavior that changes ten orders of magnitude, suggesting that it is an ideal
insulator that is immune to disorder. Non-stoichiometrically-grown SmB$_{6}$
samples also show an almost identical thermally activated behavior. At low
temperatures, however, these samples show a mysterious high bulk resistivity
plateau, which may arise from extended defect conduction in a 3D TI.
