INVESTMENT IN TRANSMISSION AFFILIATES - CHANGES TO INVESTMENTS (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Changes to investments in transmission affiliates
Add: Earnings from equity method investment	$ 51.9	$ 46.8	$ 105.5	$ 91.6
Add: Capital contributions			87.8	30.3
Investment in transmission affiliates, balance at end of period	2,284.6	2,138.1	2,284.6	2,138.1
Transmission Affiliates
Changes to investments in transmission affiliates
Investment in transmission affiliates, balance at beginning of period	2,149.0	2,027.1	2,108.9	2,005.9
Add: Earnings from equity method investment	51.9	46.8	105.5	91.6
Add: Capital contributions	45.5	18.2	87.8	30.3
Less: Distributions	46.3	36.3	102.1	72.0
Investment in transmission affiliates, balance at end of period	$ 2,200.1	2,055.8	$ 2,200.1	2,055.8
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	$ 2,121.6	2,001.6	$ 2,085.1	1,980.8
Add: Earnings from equity method investment	50.3	46.2	100.3	90.6
Add: Capital contributions	45.5	18.2	87.8	30.3
Less: Distributions	39.9	36.3	95.7	72.0
Investment in transmission affiliates, balance at end of period	$ 2,177.5	2,029.7	$ 2,177.5	2,029.7
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	$ 27.4	25.5	$ 23.8	25.1
Add: Earnings from equity method investment	1.6	0.6	5.2	1.0
Add: Capital contributions	0.0	0.0	0.0	0.0
Less: Distributions	6.4	0.0	6.4	0.0
Investment in transmission affiliates, balance at end of period	$ 22.6	$ 26.1	$ 22.6	$ 26.1

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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