This is an American marten, also commonly known as a pine marten. Martens have very shiny and luxuriant fur and at the turn of the century, they were nearly wiped out in North American due to the fur trade. They have since made a nice comeback thanks to conservation efforts and the population is now healthy throughout much of Canada and pockets of the United States. I photographed this one in front of a wood pile just outside of the northeast entrance to Yellowstone National Park in Silver Gate, Montana.
Nikon D300 with Nikkor 200-400mm lens (at 400mm) ISO 400, f/5.6 at 1/250th of a second