INVESTMENT IN TRANSMISSION AFFILIATES - CHANGES TO INVESTMENTS (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Changes to investments in transmission affiliates
Investment in transmission affiliates, balance at beginning of period	$ 1,598.9	$ 1,513.3	$ 1,553.4	$ 1,443.9
Add: Earnings (loss) from equity method investment	28.7	41.8	61.5	83.7
Add: Capital contributions	19.6	22.9	32.4	50.5
Less: Distributions	50.7	34.0	50.7	34.0
Other	0.1			(0.1)
Investment in transmission affiliates, balance at end of period	$ 1,596.6	1,544.0	$ 1,596.6	1,544.0
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	$ 1,561.1	1,515.6	$ 1,515.8	1,443.9
Add: Earnings (loss) from equity method investment	29.8	42.8	63.2	90.5
Add: Capital contributions	18.1	15.1	30.1	39.2
Less: Distributions	50.7	34.0	50.7	34.0
Other	0.1			(0.1)
Investment in transmission affiliates, balance at end of period	1,558.4	1,539.5	1,558.4	1,539.5
Dividends receivable from ATC	$ 24.2		$ 24.2		$ 39.9	$ 35.2
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	$ 37.8	(2.3)	$ 37.6	0.0
Add: Earnings (loss) from equity method investment	(1.1)	(1.0)	(1.7)	(6.8)
Add: Capital contributions	1.5	7.8	2.3	11.3
Less: Distributions	0.0	0.0	0.0	0.0
Other	0.0			0.0
Investment in transmission affiliates, balance at end of period	$ 38.2	$ 4.5	$ 38.2	$ 4.5

Carrying amount as of the balance sheet date of dividends declared but not received.

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