INVESTMENT IN TRANSMISSION AFFILIATES - SUMMARIZED FINANCIAL DATA (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021	Dec. 31, 2021
Summarized financial data
Operating revenues	$ 2,908.1	$ 2,691.4
Operating expenses	2,176.7	2,062.1
Other expense, net	36.3	44.1
Current assets	2,562.6		$ 2,656.7
Noncurrent assets	36,411.9		36,331.8
Total assets	38,974.5		38,988.5
Current liabilities	3,264.4		3,753.0
Other noncurrent liabilities	1,173.0		1,203.2
Total liabilities and members' equity	38,974.5		38,988.5
ATC
Summarized financial data
Operating revenues	191.0	188.7
Operating expenses	95.5	95.1
Other expense, net	28.0	28.5
Net income	67.5	$ 65.1
Current assets	95.5		89.8
Noncurrent assets	5,706.8		5,628.1
Total assets	5,802.3		5,717.9
Current liabilities	421.0		436.9
Long-term debt	2,562.3		2,513.0
Other noncurrent liabilities	424.7		422.0
Members' equity	2,394.3		2,346.0
Total liabilities and members' equity	$ 5,802.3		$ 5,717.9