INVESTMENT IN TRANSMISSION AFFILIATES - SUMMARIZED FINANCIAL DATA (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Summarized financial data
Total operating revenues	$ 2,691.4	$ 2,108.6
Operating expenses	2,062.1	1,482.0
Other expense, net	44.1	84.0
Current assets	2,355.5		$ 2,083.0
Noncurrent assets	35,245.1		34,945.1
Total assets	37,600.6		37,028.1
Current liabilities	3,714.9		4,148.1
Other noncurrent liabilities	1,236.3		1,229.4
Total liabilities and members' equity	37,600.6		37,028.1
ATC
Summarized financial data
Total operating revenues	188.7	186.8
Operating expenses	95.1	95.2
Other expense, net	28.5	28.5
Net income	65.1	$ 63.1
Current assets	90.0		92.7
Noncurrent assets	5,431.8		5,400.6
Total assets	5,521.8		5,493.3
Current liabilities	420.1		310.8
Long-term debt	2,412.5		2,512.2
Other noncurrent liabilities	385.0		378.2
Members' equity	2,304.2		2,292.1
Total liabilities and members' equity	$ 5,521.8		$ 5,493.3