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		2,500
Book value of net assets transferred (including the related deferred income tax liabilities) | $	$ 61.1
Utility segment
Related parties
Number of customers		27,500
Electric distribution
Related parties
Number of customers		50