INVESTMENT IN TRANSMISSION AFFILIATES - SUMMARIZED FINANCIAL DATA (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023	Dec. 31, 2023
Summarized financial data
Operating revenues	$ 1,772.0	$ 1,830.0	$ 4,452.2	$ 4,718.1
Operating expenses	1,407.2	1,404.7	3,274.0	3,623.5
Other expense, net	113.2	86.8	216.3	174.4
Current assets	2,584.1		2,584.1		$ 2,795.7
Noncurrent assets	41,998.1		41,998.1		41,144.0
Total assets	44,582.2		44,582.2		43,939.7
Current liabilities	3,491.6		3,491.6		5,114.8
Other noncurrent liabilities	794.2		794.2		835.3
Total liabilities and members' equity	44,582.2		44,582.2		43,939.7
ATC
Summarized financial data
Operating revenues	218.3	203.8	430.2	404.2
Operating expenses	109.2	101.5	214.0	200.6
Other expense, net	35.8	32.9	71.0	65.4
Net income	73.3	$ 69.4	145.2	$ 138.2
Current assets	146.5		146.5		115.2
Noncurrent assets	6,539.3		6,539.3		6,337.0
Total assets	6,685.8		6,685.8		6,452.2
Current liabilities	586.0		586.0		495.9
Long-term debt	2,810.5		2,810.5		2,736.0
Other noncurrent liabilities	573.3		573.3		585.2
Members' equity	2,716.0		2,716.0		2,635.1
Total liabilities and members' equity	$ 6,685.8		$ 6,685.8		$ 6,452.2