INVESTMENT IN TRANSMISSION AFFILIATES - CHANGES TO INVESTMENTS (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Changes to investments in transmission affiliates
Investment in transmission affiliates, balance at beginning of period	$ 1,717.7	$ 1,670.6	$ 1,720.8	$ 1,665.3
Add: Earnings (loss) from equity method investment	52.9	36.9	92.7	73.0
Add: Capital contributions	6.0	18.5	9.0	21.9
Less: Distributions	32.0	29.4	72.6	63.6
Less: Return of capital			5.3
Other	0.1	0.1	0.1	0.1
Investment in transmission affiliates, balance at end of period	$ 1,744.7	1,696.5	$ 1,744.7	1,696.5
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	$ 1,686.7	1,630.6	$ 1,684.7	1,625.3
Add: Earnings (loss) from equity method investment	52.5	37.4	92.1	73.9
Add: Capital contributions	6.0	18.1	9.0	21.1
Less: Distributions	32.0	29.4	72.6	63.6
Less: Return of capital			0.0
Other	0.1	0.1	0.1	0.1
Investment in transmission affiliates, balance at end of period	$ 1,713.3	1,656.6	$ 1,713.3	1,656.6
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	$ 31.0	40.0	$ 36.1	40.0
Add: Earnings (loss) from equity method investment	0.4	(0.5)	0.6	(0.9)
Add: Capital contributions	0.0	0.4	0.0	0.8
Less: Distributions	0.0	0.0	0.0	0.0
Less: Return of capital			5.3
Other	0.0	0.0	0.0	0.0
Investment in transmission affiliates, balance at end of period	$ 31.4	$ 39.9	$ 31.4	$ 39.9

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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