INVESTMENT IN TRANSMISSION AFFILIATES - CHANGES TO INVESTMENTS (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Changes to investments in transmission affiliates
Add: Earnings from equity method investment	$ 54.8	$ 46.7	$ 160.3	$ 138.3
Add: Capital contributions			142.4	45.5
Investment in transmission affiliates, balance at end of period	2,341.4	2,163.8	2,341.4	2,163.8
Transmission Affiliates
Changes to investments in transmission affiliates
Investment in transmission affiliates, balance at beginning of period	2,200.1	2,055.8	2,108.9	2,005.9
Add: Earnings from equity method investment	54.8	46.7	160.3	138.3
Add: Capital contributions	54.6	15.2	142.4	45.5
Less: Distributions	40.7	37.0	142.8	109.0
Add: Other		0.1		0.1
Investment in transmission affiliates, balance at end of period	$ 2,268.8	2,080.8	$ 2,268.8	2,080.8
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	$ 2,177.5	2,029.7	$ 2,085.1	1,980.8
Add: Earnings from equity method investment	54.2	46.1	154.5	136.7
Add: Capital contributions	54.6	15.2	142.4	45.5
Less: Distributions	40.7	37.0	136.4	109.0
Add: Other		0.1		0.1
Investment in transmission affiliates, balance at end of period	$ 2,245.6	2,054.1	$ 2,245.6	2,054.1
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	$ 22.6	26.1	$ 23.8	25.1
Add: Earnings from equity method investment	0.6	0.6	5.8	1.6
Add: Capital contributions	0.0	0.0	0.0	0.0
Less: Distributions	0.0	0.0	6.4	0.0
Add: Other		0.0		0.0
Investment in transmission affiliates, balance at end of period	$ 23.2	$ 26.7	$ 23.2	$ 26.7

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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