RELATED PARTIES - UMERC (Details) - UMERC $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($)	Mar. 31, 2019 MW	Jan. 01, 2017 mile customer
Related parties
Capacity Of Generation Unit | MW		187
UMERC transfer
Related parties
Miles of electric distribution lines transferred | mile			600
Miles of gas distribution mains transferred | mile			100
Transfer of net assets to affiliate | $	$ 20.6
UMERC transfer | Electricity
Related parties
Number of customers | customer			9,000
UMERC transfer | Natural Gas
Related parties
Number of customers | customer			5,300