General Montana-Dakota provides electric service at retail, serving 142,901 residential, commercial, industrial and municipal customers in 178 communities and adjacent rural areas as of December 31, 2017. For more information on the retail customer classes served, see the table below. The principal properties owned by Montana-Dakota for use in its electric operations include interests in 16 electric generating units at 11 facilities and three small portable diesel generators, as further described under System Supply, System Demand and Competition, approximately 3,200 and 4,900 miles of transmission and distribution lines, respectively, and 73 transmission and 296 distribution substations. Montana-Dakota has obtained and holds, or is in the process of renewing, valid and existing franchises authorizing it to conduct its electric operations in all of the municipalities it serves where such franchises are required. Montana-Dakota intends to protect its service area and seek renewal of all expiring franchises. At December 31, 2017, Montana-Dakota's net electric plant investment was $1.4 billion, and rate base was $1.1 billion.