INVESTMENT IN TRANSMISSION AFFILIATES - SUMMARIZED FINANCIAL DATA (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024	Dec. 31, 2024
Summarized financial data
Operating revenues	$ 2,009.5	$ 1,772.0	$ 5,159.0	$ 4,452.2
Operating expenses	1,604.6	1,407.2	3,816.6	3,274.0
Other expense, net	142.4	113.2	293.7	216.3
Current assets	2,599.1		2,599.1		$ 2,911.7
Noncurrent assets	45,925.2		45,925.2		44,451.5
Total assets	48,524.3	44,582.2	48,524.3	44,582.2	47,363.2
Current liabilities	4,697.6		4,697.6		4,841.9
Long-term debt	17,110.4		17,110.4		17,178.1
Other noncurrent liabilities	917.0		917.0		838.1
Total liabilities and members' equity	48,524.3		48,524.3		47,363.2
ATC
Summarized financial data
Operating revenues	241.2	218.3	476.1	430.2
Operating expenses	117.6	109.2	234.3	214.0
Other expense, net	42.7	35.8	81.8	71.0
Net income	80.9	$ 73.3	160.0	$ 145.2
Current assets	151.6		151.6		126.6
Noncurrent assets	7,143.3		7,143.3		6,792.6
Total assets	7,294.9		7,294.9		6,919.2
Current liabilities	735.8		735.8		482.4
Long-term debt	3,033.0		3,033.0		3,083.4
Other noncurrent liabilities	565.0		565.0		545.0
Members' equity	2,961.1		2,961.1		2,808.4
Total liabilities and members' equity	$ 7,294.9		$ 7,294.9		$ 6,919.2