While Jefferson Lines is a good company and one of the oldest interstate companies in the nation there is also another option for going from Miles City to points west on I-94 and I-90 to the Idaho border and east to Glendive, Sidney, and Williston (Connections to Greyhound can currently be made in Missoula and Sidney.)
R & R Stagelines.
R & R Stagelines is a a new (established last year) Montana-based company and a federally authorized carrier. They have a fledgling website however it it is best to call them to book your ride.
Their number is 406-777-7323.