INVESTMENT IN TRANSMISSION AFFILIATES - SUMMARIZED FINANCIAL DATA (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021	Dec. 31, 2021
Summarized financial data
Operating revenues	$ 2,003.0	$ 1,746.5	$ 7,039.0	$ 6,114.1
Operating expenses	1,598.8	1,356.5	5,495.2	4,726.0
Other expense, net	29.1	50.5	122.4	133.6
Current assets	2,771.0		2,771.0		$ 2,656.7
Noncurrent assets	37,990.0		37,990.0		36,331.8
Total assets	40,761.0		40,761.0		38,988.5
Current liabilities	3,369.7		3,369.7		3,753.0
Other noncurrent liabilities	1,323.0		1,323.0		1,203.2
Total liabilities and members' equity	40,761.0		40,761.0		38,988.5
ATC
Summarized financial data
Operating revenues	169.8	186.8	552.4	561.4
Operating expenses	97.7	91.3	288.4	278.8
Other expense, net	34.5	28.6	91.3	85.2
Net income	37.6	$ 66.9	172.7	$ 197.4
Current assets	95.9		95.9		89.8
Noncurrent assets	5,901.8		5,901.8		5,628.1
Total assets	5,997.7		5,997.7		5,717.9
Current liabilities	498.4		498.4		436.9
Long-term debt	2,612.3		2,612.3		2,513.0
Other noncurrent liabilities	464.2		464.2		422.0
Members' equity	2,422.8		2,422.8		2,346.0
Total liabilities and members' equity	$ 5,997.7		$ 5,997.7		$ 5,717.9