INVESTMENT IN TRANSMISSION AFFILIATES - CHANGES TO INVESTMENTS (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2017
Changes to investments in transmission affiliates
Investment in transmission affiliates, balance at beginning of period	$ 1,665.3	$ 1,553.4
Add: Earnings (loss) from equity method investment	36.1	32.8
Add: Capital contributions	3.4	12.8
Less: Distributions	34.2
Less: Other		0.1
Investment in transmission affiliates, balance at end of period	$ 1,670.6	1,598.9
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	$ 1,625.3	1,515.8
Add: Earnings (loss) from equity method investment	36.5	33.4
Add: Capital contributions	3.0	12.0
Less: Distributions	34.2
Less: Other		0.1
Investment in transmission affiliates, balance at end of period	$ 1,630.6	1,561.1
Dividends receivable from ATC			$ 39.9
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	$ 40.0	37.6
Add: Earnings (loss) from equity method investment	(0.4)	(0.6)
Add: Capital contributions	0.4	0.8
Less: Distributions	0.0
Less: Other		0.0
Investment in transmission affiliates, balance at end of period	$ 40.0	$ 37.8

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

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