Mont. Code § 69-3-109
The commission may, in its discretion, investigate and ascertain the value of the property of each public utility actually used and useful for the convenience of the public. The commission is not bound to accept or use any particular value in determining rates. However, if any value is used, the value may not exceed the original cost of the property, except that the commission may include all or some of an acquisition adjustment for certain property purchased by a public utility in the purchasing utility's rate base if the transfer of the property to the purchasing utility is in the public interest. In making the investigation, the commission may avail itself of all information contained in the assessment rolls of various counties or in the public records of the various branches of the state government or of any other information obtainable, and the commission may at any time on its own initiative make a revaluation of the property.
§ 69-3-109, MCA