S56 TRACON Controller reported air carrier descended below MVA assigned altitude resulted in a CFTT event.

Narrative

I was working an arrival sector and told Aircraft X to expect Runway 17 and to descend and maintain 110 feet. He read back 110 and began descending. He was descending to 110 direct UDUZU to join Runway 17 localizer. He descended below 110 and I caught it at 107 and told him to maintain 110. I noticed he was still descending even after I issued 110 and he responded with 'what altitude was that? He ultimately descended to 102 or 103 in an 110 foot MVA. I issued a low altitude alert and told him to climb and maintain 110 and the MVA in that area was 110. He responded with 'sorry I think I might have read that back wrong.' He climbed to 110 and continued to UDUZU where the next controller issued the approach clearance. He landed without issue. Recommendation:Even though the readback was correct and this was a pilot error it is still necessary to maintain focus while working the position. You never know when a pilot might make a mistake that could be dangerous. We are all human and make mistakes. But this was just a matter of recognizing the mistake and correcting it before it became dangerous. The dangerous factor was that Aircraft X was flying over mountains so the MVA was very important in that area.