Mont. Code Ann. § 33-23-217

If an insurer cancels a motor vehicle policy covering a commercial or private motor vehicle for nonpayment of a premium, the insurer shall process the cancellation on a pro rata basis. The insurer shall return whatever gross unearned premium is due under the motor vehicle policy to the insured. For purposes of this section, the term "unearned premium" means that part of the original premium, including deposits, not yet earned by the insurer and therefore due the insured if the policy is canceled.