INVESTMENT IN TRANSMISSION AFFILIATES - CHANGES TO INVESTMENTS (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2021	Mar. 31, 2020
Changes to investments in transmission affiliates
Add: Earnings from equity method investment	$ 42.6	$ 39.8
Add: Capital contributions	0.0	3.0
Transmission Affiliates
Changes to investments in transmission affiliates
Investment in transmission affiliates, balance at beginning of period	1,764.3	1,720.8
Add: Earnings from equity method investment	42.6	39.8
Add: Capital contributions		3.0
Less: Distributions	33.4	40.6
Less: Return of capital		5.3
Add: Other	0.1
Investment in transmission affiliates, balance at end of period	$ 1,773.6	1,717.7
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	$ 1,733.5	1,684.7
Add: Earnings from equity method investment	41.7	39.6
Add: Capital contributions		3.0
Less: Distributions	33.4	40.6
Less: Return of capital		0.0
Add: Other	0.1
Investment in transmission affiliates, balance at end of period	$ 1,741.9	1,686.7
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	$ 30.8	36.1
Add: Earnings from equity method investment	0.9	0.2
Add: Capital contributions		0.0
Less: Distributions	0.0	0.0
Less: Return of capital		5.3
Add: Other	0.0
Investment in transmission affiliates, balance at end of period	$ 31.7	$ 31.0

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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