Mont. Code Ann. § 17-5-1628

Notwithstanding the restrictions of any other law, all banks, trust companies, savings banks and institutions, building and loan associations, savings and loan associations, investment companies, and other persons carrying on a banking business, all insurance companies, insurance associations, and other persons carrying on an insurance business, and all executors, administrators, guardians, trustees, and other fiduciaries may legally invest debt service funds, money, or other funds belonging to them or within their control in bonds or notes issued under this part.