INVESTMENT IN TRANSMISSION AFFILIATES - SUMMARIZED FINANCIAL DATA (Details) - ATC - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Income statement data
Operating revenues	$ 182.2	$ 165.5	$ 359.9	$ 330.9
Operating expenses	93.6	91.5	184.0	176.4
Other expense, net	28.6	25.4	57.4	53.0
Net income	60.0	$ 48.6	118.5	$ 101.5
Balance sheet data
Current assets	88.8		88.8		$ 87.2
Noncurrent assets	5,100.7		5,100.7		4,928.8
Total assets	5,189.5		5,189.5		5,016.0
Current liabilities	562.1		562.1		640.0
Long-term debt	2,213.0		2,213.0		2,014.0
Other noncurrent liabilities	294.4		294.4		295.3
Shareholders' equity	2,120.0		2,120.0		2,066.7
Total liabilities and shareholders' equity	$ 5,189.5		$ 5,189.5		$ 5,016.0