INVESTMENT IN TRANSMISSION AFFILIATES - CHANGES TO INVESTMENTS (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Changes to investments in transmission affiliates
Add: Earnings from equity method investment	$ 43.6	$ 43.0	$ 87.4	$ 84.7
Add: Capital contributions			33.3	30.3
Transmission Affiliates
Changes to investments in transmission affiliates
Investment in transmission affiliates, balance at beginning of period	1,921.7	1,818.2	1,909.2	1,789.4
Add: Earnings from equity method investment	43.6	43.0	87.4	84.7
Add: Capital contributions	27.2	9.2	33.3	30.3
Less: Distributions	36.6	33.2	74.0	67.2
Investment in transmission affiliates, balance at end of period	$ 1,955.9	1,837.2	$ 1,955.9	1,837.2
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	$ 1,896.2	1,795.0	$ 1,884.6	1,766.9
Add: Earnings from equity method investment	43.1	42.6	86.0	83.6
Add: Capital contributions	27.2	9.2	33.3	30.3
Less: Distributions	34.7	33.2	72.1	67.2
Investment in transmission affiliates, balance at end of period	$ 1,931.8	1,813.6	$ 1,931.8	1,813.6
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	$ 25.5	23.2	$ 24.6	22.5
Add: Earnings from equity method investment	0.5	0.4	1.4	1.1
Add: Capital contributions	0.0	0.0	0.0	0.0
Less: Distributions	1.9	0.0	1.9	0.0
Investment in transmission affiliates, balance at end of period	$ 24.1	$ 23.6	$ 24.1	$ 23.6

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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