INVESTMENT IN TRANSMISSION AFFILIATES - CHANGES TO INVESTMENTS (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Changes to investments in transmission affiliates
Add: Earnings from equity method investment	$ 44.7	$ 63.7	$ 132.1	$ 148.4
Add: Capital contributions			51.5	39.4
Transmission Affiliates
Changes to investments in transmission affiliates
Investment in transmission affiliates, balance at beginning of period	1,955.9	1,837.2	1,909.2	1,789.4
Add: Earnings from equity method investment	44.7	63.7	132.1	148.4
Add: Capital contributions	18.2	9.1	51.5	39.4
Less: Distributions	35.0	34.1	109.0	101.3
Investment in transmission affiliates, balance at end of period	$ 1,983.8	1,875.9	$ 1,983.8	1,875.9
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	$ 1,931.8	1,813.6	$ 1,884.6	1,766.9
Add: Earnings from equity method investment	44.2	63.2	130.2	146.8
Add: Capital contributions	18.2	9.1	51.5	39.4
Less: Distributions	35.0	34.1	107.1	101.3
Investment in transmission affiliates, balance at end of period	$ 1,959.2	1,851.8	$ 1,959.2	1,851.8
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	$ 24.1	23.6	$ 24.6	22.5
Add: Earnings from equity method investment	0.5	0.5	1.9	1.6
Add: Capital contributions	0.0	0.0	0.0	0.0
Less: Distributions	0.0	0.0	1.9	0.0
Investment in transmission affiliates, balance at end of period	$ 24.6	$ 24.1	$ 24.6	$ 24.1

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