RELATED PARTIES - UMERC (Details) - UMERC transfer $ in Millions	6 Months Ended
	Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($)	Jan. 01, 2017 customer mile
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	$ 0.0
Utility segment
Related parties
Number of customers			27,500
Electric distribution
Related parties
Number of customers			50