INVESTMENT IN TRANSMISSION AFFILIATES - CHANGES TO INVESTMENTS (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2024	Mar. 31, 2023
Changes to investments in transmission affiliates
Add: Earnings from equity method investment	$ 44.8	$ 43.8
Add: Capital contributions	12.1	6.1
Transmission Affiliates
Changes to investments in transmission affiliates
Investment in transmission affiliates, balance at beginning of period	2,005.9	1,909.2
Add: Earnings from equity method investment	44.8	43.8
Add: Capital contributions	12.1	6.1
Less: Distributions	35.7	37.4
Investment in transmission affiliates, balance at end of period	$ 2,027.1	1,921.7
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	$ 1,980.8	1,884.6
Add: Earnings from equity method investment	44.4	42.9
Add: Capital contributions	12.1	6.1
Less: Distributions	35.7	37.4
Investment in transmission affiliates, balance at end of period	$ 2,001.6	1,896.2
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	$ 25.1	24.6
Add: Earnings from equity method investment	0.4	0.9
Add: Capital contributions	0.0	0.0
Less: Distributions	0.0	0.0
Investment in transmission affiliates, balance at end of period	$ 25.5	$ 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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