INVESTMENT IN TRANSMISSION AFFILIATES - CHANGES TO INVESTMENTS (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Changes to investments in transmission affiliates
Add: Earnings from equity method investment	$ 46.8	$ 43.6	$ 91.6	$ 87.4
Add: Capital contributions			30.3	33.3
Transmission Affiliates
Changes to investments in transmission affiliates
Investment in transmission affiliates, balance at beginning of period	2,027.1	1,921.7	2,005.9	1,909.2
Add: Earnings from equity method investment	46.8	43.6	91.6	87.4
Add: Capital contributions	18.2	27.2	30.3	33.3
Less: Distributions	36.3	36.6	72.0	74.0
Investment in transmission affiliates, balance at end of period	$ 2,055.8	1,955.9	$ 2,055.8	1,955.9
ATC
Investment in transmission affiliates
Equity method investment, ownership interest (as a percent)	60.00%		60.00%
Changes to investments in transmission affiliates
Investment in transmission affiliates, balance at beginning of period	$ 2,001.6	1,896.2	$ 1,980.8	1,884.6
Add: Earnings from equity method investment	46.2	43.1	90.6	86.0
Add: Capital contributions	18.2	27.2	30.3	33.3
Less: Distributions	36.3	34.7	72.0	72.1
Investment in transmission affiliates, balance at end of period	$ 2,029.7	1,931.8	$ 2,029.7	1,931.8
ATC Holdco
Investment in transmission affiliates
Equity method investment, ownership interest (as a percent)	75.00%		75.00%
Changes to investments in transmission affiliates
Investment in transmission affiliates, balance at beginning of period	$ 25.5	25.5	$ 25.1	24.6
Add: Earnings from equity method investment	0.6	0.5	1.0	1.4
Add: Capital contributions	0.0	0.0	0.0	0.0
Less: Distributions	0.0	1.9	0.0	1.9
Investment in transmission affiliates, balance at end of period	$ 26.1	$ 24.1	$ 26.1	$ 24.1

Amount of distribution received from equity method investee for return on investment, classified as operating activities. Excludes distribution for return of investment, classified as investing activities.

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