Indeed. Buts its not just Montana that feeds into the BIL hub (Big Sky). They serve the following locations:
Idaho: Boise, Idaho Falls
Montana: Billings, Glasgow, Glendive, Great Falls, Havre, Helena, Kalispell, Lewistown, Miles City, Missoula, Sidney, Wolf Point
North Dakota: Bismarck, Williston
Washington: Moses Lake, Olympia, Seattle/Tacoma, Spokane
Wyoming: Casper, Gillette
I remember even before HP express served BIL, if you looked up a fare on travelocity from say- Williston, ND to Phoenix, AZ; It would route you through ISN-SDY-BIL-DEN (Big Sky) then from DEN-PHX (HP mainline). Lol.. glad its more direct now...