INVESTMENT IN TRANSMISSION AFFILIATES - SUMMARIZED FINANCIAL DATA (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020
Summarized financial data
Operating revenues	$ 1,676.2	$ 1,548.7	$ 4,367.6	$ 3,657.3
Operating expenses	1,307.4	1,209.9	3,369.5	2,691.9
Other expense, net	39.0	42.9	83.1	126.9
Current assets	2,278.1		2,278.1		$ 2,083.0
Noncurrent assets	35,516.8		35,516.8		34,945.1
Total assets	37,794.9		37,794.9		37,028.1
Current liabilities	3,373.9		3,373.9		4,148.1
Other noncurrent liabilities	1,256.7		1,256.7		1,229.4
Total liabilities and members' equity	37,794.9		37,794.9		37,028.1
ATC
Summarized financial data
Operating revenues	185.9	203.1	374.6	389.9
Operating expenses	92.4	97.5	187.5	192.7
Other expense, net	28.1	25.4	56.6	53.9
Net income	65.4	$ 80.2	130.5	$ 143.3
Current assets	92.5		92.5		92.7
Noncurrent assets	5,462.6		5,462.6		5,400.6
Total assets	5,555.1		5,555.1		5,493.3
Current liabilities	427.5		427.5		310.8
Long-term debt	2,412.7		2,412.7		2,512.2
Other noncurrent liabilities	397.4		397.4		378.2
Members' equity	2,317.5		2,317.5		2,292.1
Total liabilities and members' equity	$ 5,555.1		$ 5,555.1		$ 5,493.3