INVESTMENT IN TRANSMISSION AFFILIATES - SUMMARIZED FINANCIAL DATA (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Income statement data
Operating revenues	$ 2,108.6	$ 2,377.4
Operating expenses	1,482.0	1,834.6
Other expense, net	84.0	57.4
Balance sheet data
Current assets	1,845.3		$ 2,093.6
Noncurrent assets	32,986.9		32,858.2
Total assets	34,832.2		34,951.8
Current liabilities	2,847.7		3,182.7
Other noncurrent liabilities	1,106.0		1,128.9
Total liabilities and shareholders' equity	34,832.2		34,951.8
ATC
Income statement data
Operating revenues	186.8	177.7
Operating expenses	95.2	90.4
Other expense, net	28.5	28.8
Net income	63.1	$ 58.5
Balance sheet data
Current assets	82.4		84.7
Noncurrent assets	5,283.2		5,244.2
Total assets	5,365.6		5,328.9
Current liabilities	526.3		502.6
Long-term debt	2,313.0		2,312.8
Other noncurrent liabilities	307.9		298.9
Shareholders' equity	2,218.4		2,214.6
Total liabilities and shareholders' equity	$ 5,365.6		$ 5,328.9