INVESTMENT IN TRANSMISSION AFFILIATES - CHANGES TO INVESTMENTS (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Changes to investments in transmission affiliates
Add: Earnings from equity method investment	$ 46.7	$ 44.7	$ 138.3	$ 132.1
Add: Capital contributions			45.5	51.5
Transmission Affiliates
Changes to investments in transmission affiliates
Investment in transmission affiliates, balance at beginning of period	2,055.8	1,955.9	2,005.9	1,909.2
Add: Earnings from equity method investment	46.7	44.7	138.3	132.1
Add: Capital contributions	15.2	18.2	45.5	51.5
Less: Distributions	37.0	35.0	109.0	109.0
Add (less): Other	0.1		0.1
Investment in transmission affiliates, balance at end of period	$ 2,080.8	1,983.8	$ 2,080.8	1,983.8
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	$ 2,029.7	1,931.8	$ 1,980.8	1,884.6
Add: Earnings from equity method investment	46.1	44.2	136.7	130.2
Add: Capital contributions	15.2	18.2	45.5	51.5
Less: Distributions	37.0	35.0	109.0	107.1
Add (less): Other	0.1		0.1
Investment in transmission affiliates, balance at end of period	$ 2,054.1	1,959.2	$ 2,054.1	1,959.2
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	$ 26.1	24.1	$ 25.1	24.6
Add: Earnings from equity method investment	0.6	0.5	1.6	1.9
Add: Capital contributions	0.0	0.0	0.0	0.0
Less: Distributions	0.0	0.0	0.0	1.9
Add (less): Other	0.0		0.0
Investment in transmission affiliates, balance at end of period	$ 26.7	$ 24.6	$ 26.7	$ 24.6

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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