INVESTMENT IN TRANSMISSION AFFILIATES - CHANGES TO INVESTMENTS (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2025	Mar. 31, 2024
Changes to investments in transmission affiliates
Add: Earnings from equity method investment	$ 53.6	$ 44.8
Add: Capital contributions	42.3	12.1
Investment in transmission affiliates, balance at end of period	2,233.9	2,106.8
Transmission Affiliates
Changes to investments in transmission affiliates
Investment in transmission affiliates, balance at beginning of period	2,108.9	2,005.9
Add: Earnings from equity method investment	53.6	44.8
Add: Capital contributions	42.3	12.1
Less: Distributions	55.8	35.7
Investment in transmission affiliates, balance at end of period	$ 2,149.0	2,027.1
ATC
Investment in transmission affiliates
Equity method investment, ownership interest (as a percent)	60.00%
Changes to investments in transmission affiliates
Investment in transmission affiliates, balance at beginning of period	$ 2,085.1	1,980.8
Add: Earnings from equity method investment	50.0	44.4
Add: Capital contributions	42.3	12.1
Less: Distributions	55.8	35.7
Investment in transmission affiliates, balance at end of period	$ 2,121.6	2,001.6
ATC Holdco
Investment in transmission affiliates
Equity method investment, ownership interest (as a percent)	75.00%
Changes to investments in transmission affiliates
Investment in transmission affiliates, balance at beginning of period	$ 23.8	25.1
Add: Earnings from equity method investment	3.6	0.4
Add: Capital contributions	0.0	0.0
Less: Distributions	0.0	0.0
Investment in transmission affiliates, balance at end of period	$ 27.4	$ 25.5

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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