Report 1533: Motorist & passenger see white creature standing off I-15

While returning to Helena after a trip to Great Falls, I saw....something....illuminated in the cars headlights on a moutainside approximately 150 feet or less off of I-15. Trying to interpret what i saw, i didn't say anything at first. Then a few seconds later my girlfriend said, "do you believe in bigfoot?"  It literally sent a chill down my spine and all I could think of to say was, "You saw that too...." . She simply said, "yes". I don't know what it was, but i know what it wasn't. It definitely wasn't a bear, goat, sheep, deer, elk, lion, dog, or any other wildlife that lives around here. It was white, standing erect, in excess of 6' tall and appeared to be just watching traffic on the highway (hard to say really, because the duration of this sighting was only a few seconds and it was dark) I find it hard to believe that someone was out there late at night in freezing weather standing in the snow in the middle of nowhere in a gorilla suit (especially a white gorilla suit!) so I've ruled that out as well.