I knew that McMullin did well in Idaho and that his best areas in Utah were in large counties, but I was still surprised to see an Idaho county top.
It makes sense that some lot of states feature towards the bottom of all lists given both the size of counties and the number of them in those states.
Yes, Madison County, Idaho is all-but-completely Mormon. Rexburg is home to a Brigham Young University, and as we can recall, Mormons were not-so-forgiving of Donald Trump's moral bankruptcy unlike other "Christians" who buried their hands in the sand because muh Supreme Court justices.
I know in my state, Daviess County (McMullin and Castle's best county) has the largest Amish presence in the state. Not sure if they are politically active, though, or how much if so.