SEGMENT INFORMATION - Narrative (Details) $ in Millions	12 Months Ended
	Dec. 31, 2018 USD ($) company segment	Dec. 31, 2017 USD ($)
Segment Reporting Information [Line Items]
Number of companies that transmit hydro electricity | company	2
Amount of natural gas transmission purchased	$ 62.5
Number of reportable segments | segment	1
Transmission Merger Cost Recovery
Segment Reporting Information [Line Items]
Amount of recovery through transmission rates		$ 27.5