INVESTMENT IN TRANSMISSION AFFILIATES - CHANGES TO INVESTMENTS (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2022	Mar. 31, 2021
Changes to investments in transmission affiliates
Add: Earnings from equity method investment	$ 41.7	$ 42.6
Add: Capital contributions	21.1	0.0
Transmission Affiliates
Changes to investments in transmission affiliates
Investment in transmission affiliates, balance at beginning of period	1,789.4	1,764.3
Add: Earnings from equity method investment	41.7	42.6
Add: Capital contributions	21.1
Less: Distributions	34.0	33.4
Add: Other		(0.1)
Investment in transmission affiliates, balance at end of period	$ 1,818.2	1,773.6
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	$ 1,766.9	1,733.5
Add: Earnings from equity method investment	41.0	41.7
Add: Capital contributions	21.1
Less: Distributions	34.0	33.4
Add: Other		(0.1)
Investment in transmission affiliates, balance at end of period	$ 1,795.0	1,741.9
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	$ 22.5	30.8
Add: Earnings from equity method investment	0.7	0.9
Add: Capital contributions	0.0
Less: Distributions	0.0	0.0
Add: Other		0.0
Investment in transmission affiliates, balance at end of period	$ 23.2	$ 31.7

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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