Idaho Code § 61-508
Whenever the commission, after a hearing had upon its own motion or upon complaint, shall find that additions, extensions, repairs or improvements to or changes in the existing plant, scales, equipment, apparatus, facilities or other physical property of any public utility or of any two (2) or more public utilities ought reasonably to be made, or that a new structure or structures should be erected, to promote the security or convenience of its employees or the public, or in any other way to secure adequate service or facilities, the commission shall make and serve an order directing such additions, extensions, repairs, improvements, or changes be made or such structure or structures be erected in the manner and within the time specified in said order. If any additions, extensions, repairs, improvements or changes, or any new structure or structures which the commission has ordered to be erected, requires joint action by two (2) or more public utilities the commission shall notify the said public utilities that such additions, extensions, repairs, improvements or changes or new structure or structures have been ordered and that the same shall be made at the joint cost, whereupon the said public utilities shall have such reasonable time as the commission may grant within which to agree upon the portion or division of cost of such additions, extensions, repairs, improvements or changes or new structure or structures, which each shall bear. If at the expiration of such time, such public utilities shall fail to file with the commission a statement that an agreement has been made for a division or apportionment of the cost or expense of such additions, extensions, repairs, improvements or changes, or new structures or structure, the commission shall have authority, after further hearing, to make an order fixing the proportion of such cost or expense to be borne by each public utility and the manner in which the same shall be paid or secured.
Idaho Code § 61-508