INVESTMENT IN TRANSMISSION AFFILIATES - SUMMARIZED FINANCIAL DATA (Details) - ATC - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Income statement data
Operating revenues	$ 177.7	$ 165.4
Operating expenses	90.4	84.9
Other expense, net	28.8	27.6
Net income	58.5	$ 52.9
Balance sheet data
Current assets	87.8		$ 87.2
Noncurrent assets	5,012.7		4,928.8
Total assets	5,100.5		5,016.0
Current liabilities	571.1		640.0
Long-term debt	2,164.1		2,014.0
Other noncurrent liabilities	288.6		295.3
Shareholders' equity	2,076.7		2,066.7
Total liabilities and shareholders' equity	$ 5,100.5		$ 5,016.0