INVESTMENT IN TRANSMISSION AFFILIATES - CHANGES TO INVESTMENTS (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Changes to investments in transmission affiliates
Add: Earnings from equity method investment	$ 41.3	$ 52.9	$ 83.9	$ 92.7
Add: Capital contributions			0.0	9.0
Transmission Affiliates
Changes to investments in transmission affiliates
Investment in transmission affiliates, balance at beginning of period	1,773.6	1,717.7	1,764.3	1,720.8
Add: Earnings from equity method investment	41.3	52.9	83.9	92.7
Add: Capital contributions		6.0		9.0
Less: Distributions	32.8	32.0	66.2	72.6
Less: Return of capital				5.3
Add (less): Other	(0.1)	0.1		0.1
Investment in transmission affiliates, balance at end of period	$ 1,782.0	1,744.7	$ 1,782.0	1,744.7
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	$ 1,741.9	1,686.7	$ 1,733.5	1,684.7
Add: Earnings from equity method investment	40.7	52.5	82.4	92.1
Add: Capital contributions		6.0		9.0
Less: Distributions	32.8	32.0	66.2	72.6
Less: Return of capital				0.0
Add (less): Other	(0.1)	0.1		0.1
Investment in transmission affiliates, balance at end of period	$ 1,749.7	1,713.3	$ 1,749.7	1,713.3
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	$ 31.7	31.0	$ 30.8	36.1
Add: Earnings from equity method investment	0.6	0.4	1.5	0.6
Add: Capital contributions		0.0		0.0
Less: Distributions	0.0	0.0	0.0	0.0
Less: Return of capital				5.3
Add (less): Other	0.0	0.0		0.0
Investment in transmission affiliates, balance at end of period	$ 32.3	$ 31.4	$ 32.3	$ 31.4

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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