RELATED PARTIES - UMERC (Details) - UMERC transfer $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($)	Jan. 01, 2017 mile customer
Related parties
Miles of electric distribution lines | mile		600
Miles of natural gas distribution mains | mile		100
Transfer of net assets to UMERC | $	$ 20.6
Electric utility
Related parties
Number of customers | customer		9,000
Natural gas utility segment
Related parties
Number of customers | customer		5,300