Not Idaho and it wasn't in Sept...but I went on an elk trip last fall and learned some things regarding clothing that I will share. Apply them as you see fit.
We were in Montana in November and the overnight lows were in the -15 to -20 range, with daytime, ahem, "highs" in the 0 to 10 above range...and I was
overdressed for the first couple of days of hunting and sweating like a pig. We were staying in wall tents that did have a wood stove in them, but when that baby ran out of wood in the middle of the night it got FROSTY quick like!

We also were "horsing" it, which I really enjoyed...except for the time that my steed tried to throw me over a cliff that is!

Also, consider your boots...and this is also from the "I am lucky to still be alive" manual...big toed, well insulated boots are a pooper when you try to get them in the stirrups! A key bit of horse wisdom...you DO NOT want your boot(s) stuck in a stirrup if you fall off and the horse keeps boogeying on...which they will!
If you are going to "run and gun" and not sit still all day, you can dress much lighter than you think. A base layer of polypropylene undies, medium weight wool pants and a good wool shirt and then maybe a wool vest...good hat, gloves and boots and you are set. The smart guys wore fanny packs only and traveled light...I, and some of you jokers, like Wapsi, can draw your own inferences here

, started off the week with my regular, decent sized backpack and then got smarter as time went on and only was wearing my fanny pack by the end of the week.
All the guides wore wool, a couple of the hunters in camp were all decked out in Sitka gear, etc. But I think it was telling that the "pros" were in plain ol' wool. I had wool too, but it was too heavy for the amount of walking that we did. Good hats(wool too), gloves and boots and you are good to go. Enjoy!