INVESTMENT IN TRANSMISSION AFFILIATES - CHANGES TO INVESTMENTS (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Changes to investments in transmission affiliates
Add: Earnings from equity method investment	$ 63.7	$ 42.3	$ 148.4	$ 126.2
Add: Capital contributions			39.4	0.0
Transmission Affiliates
Changes to investments in transmission affiliates
Investment in transmission affiliates, balance at beginning of period	1,837.2	1,782.0	1,789.4	1,764.3
Add: Earnings from equity method investment	63.7	42.3	148.4	126.2
Add: Capital contributions	9.1		39.4
Less: Distributions	34.1	33.0	101.3	99.2
Investment in transmission affiliates, balance at end of period	$ 1,875.9	1,791.3	$ 1,875.9	1,791.3
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	$ 1,813.6	1,749.7	$ 1,766.9	1,733.5
Add: Earnings from equity method investment	63.2	41.7	146.8	124.1
Add: Capital contributions	9.1		39.4
Less: Distributions	34.1	33.0	101.3	99.2
Investment in transmission affiliates, balance at end of period	$ 1,851.8	1,758.4	$ 1,851.8	1,758.4
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	$ 23.6	32.3	$ 22.5	30.8
Add: Earnings from equity method investment	0.5	0.6	1.6	2.1
Add: Capital contributions	0.0		0.0
Less: Distributions	0.0	0.0	0.0	0.0
Investment in transmission affiliates, balance at end of period	$ 24.1	$ 32.9	$ 24.1	$ 32.9

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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