INVESTMENT IN TRANSMISSION AFFILIATES - SUMMARIZED FINANCIAL DATA (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Summarized financial data
Operating revenues	$ 1,746.5	$ 1,651.0	$ 6,114.1	$ 5,308.3
Operating expenses	1,356.5	1,280.8	4,726.0	3,972.7
Other expense, net	50.5	56.2	133.6	183.1
Current assets	2,408.9		2,408.9		$ 2,083.0
Noncurrent assets	35,992.0		35,992.0		34,945.1
Total assets	38,400.9		38,400.9		37,028.1
Current liabilities	3,692.6		3,692.6		4,148.1
Other noncurrent liabilities	1,283.6		1,283.6		1,229.4
Total liabilities and members' equity	38,400.9		38,400.9		37,028.1
ATC
Summarized financial data
Operating revenues	186.8	187.8	561.4	577.7
Operating expenses	91.3	93.0	278.8	285.7
Other expense, net	28.6	28.1	85.2	82.0
Net income	66.9	$ 66.7	197.4	$ 210.0
Current assets	87.0		87.0		92.7
Noncurrent assets	5,539.8		5,539.8		5,400.6
Total assets	5,626.8		5,626.8		5,493.3
Current liabilities	483.0		483.0		310.8
Long-term debt	2,412.8		2,412.8		2,512.2
Other noncurrent liabilities	398.9		398.9		378.2
Members' equity	2,332.1		2,332.1		2,292.1
Total liabilities and members' equity	$ 5,626.8		$ 5,626.8		$ 5,493.3