RELATED PARTIES - UMERC (Details) - UMERC transfer $ in Millions		9 Months Ended
	Jan. 01, 2017 USD ($) customer mile	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)
Related parties
Miles of electric distribution lines | mile	2,500
Book value of net assets transferred (including the related deferred income tax liabilities) | $	$ 60.0	$ 60.0	$ 0.0
Utility segment
Related parties
Number of customers	27,500
Electric distribution
Related parties
Number of customers	50