INVESTMENT IN TRANSMISSION AFFILIATES - SUMMARIZED FINANCIAL DATA (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023
Summarized financial data
Operating revenues	$ 2,680.2	$ 2,888.1
Operating expenses	1,866.8	2,218.8
Other expense, net	103.1	87.6
Current assets	2,604.6		$ 2,795.7
Noncurrent assets	41,322.6		41,144.0
Total assets	43,927.2		43,939.7
Current liabilities	4,709.7		5,114.8
Other noncurrent liabilities	806.3		835.3
Total liabilities and members' equity	43,927.2		43,939.7
ATC
Summarized financial data
Operating revenues	211.9	200.4
Operating expenses	104.8	99.1
Other expense, net	35.2	32.5
Net income	71.9	$ 68.8
Current assets	133.2		115.2
Noncurrent assets	6,423.4		6,337.0
Total assets	6,556.6		6,452.2
Current liabilities	587.9		495.9
Long-term debt	2,736.3		2,736.0
Other noncurrent liabilities	562.2		585.2
Members' equity	2,670.2		2,635.1
Total liabilities and members' equity	$ 6,556.6		$ 6,452.2