INVESTMENT IN TRANSMISSION AFFILIATES - SUMMARIZED FINANCIAL DATA (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023	Dec. 31, 2023
Summarized financial data
Operating revenues	$ 1,863.5	$ 1,957.4	$ 6,315.7	$ 6,675.5
Operating expenses	1,479.8	1,485.4	4,753.8	5,108.9
Other expense, net	113.5	96.0	329.8	270.4
Current assets	2,619.2		2,619.2		$ 2,795.7
Noncurrent assets	42,577.3		42,577.3		41,144.0
Total assets	45,196.5		45,196.5		43,939.7
Current liabilities	4,009.4		4,009.4		5,114.8
Other noncurrent liabilities	822.5		822.5		835.3
Total liabilities and members' equity	45,196.5		45,196.5		43,939.7
ATC
Summarized financial data
Operating revenues	221.4	206.2	651.6	610.4
Operating expenses	110.1	102.8	324.1	303.4
Other expense, net	36.4	32.9	107.4	98.3
Net income	74.9	$ 70.5	220.1	$ 208.7
Current assets	130.0		130.0		115.2
Noncurrent assets	6,660.4		6,660.4		6,337.0
Total assets	6,790.4		6,790.4		6,452.2
Current liabilities	525.7		525.7		495.9
Long-term debt	2,933.9		2,933.9		2,736.0
Other noncurrent liabilities	573.5		573.5		585.2
Members' equity	2,757.3		2,757.3		2,635.1
Total liabilities and members' equity	$ 6,790.4		$ 6,790.4		$ 6,452.2