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INVESTMENT IN TRANSMISSION AFFILIATES - CHANGES TO INVESTMENTS (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Changes to investments in transmission affiliates
Investment in transmission affiliates, balance at beginning of period	$ 1,596.6	$ 1,544.0	$ 1,553.4	$ 1,443.9
Add: Earnings (loss) from equity method investment	33.7	39.2	95.2	122.9
Add: Capital contributions	11.3	12.8	43.7	63.3
Less: Distributions	27.8	35.2	78.5	69.2
Less: Other	0.1		0.1	0.1
Investment in transmission affiliates, balance at end of period	$ 1,613.7	1,560.8	$ 1,613.7	1,560.8
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	$ 1,558.4	1,539.5	$ 1,515.8	1,443.9
Add: Earnings (loss) from equity method investment	34.6	40.9	97.8	131.4
Add: Capital contributions	9.1	12.1	39.2	51.3
Less: Distributions	27.8	35.2	78.5	69.2
Less: Other	0.1		0.1	0.1
Investment in transmission affiliates, balance at end of period	1,574.2	1,557.3	1,574.2	1,557.3
Dividends receivable from ATC	$ 27.7		$ 27.7		$ 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	$ 38.2	4.5	$ 37.6	0.0
Add: Earnings (loss) from equity method investment	(0.9)	(1.7)	(2.6)	(8.5)
Add: Capital contributions	2.2	0.7	4.5	12.0
Less: Distributions	0.0	0.0	0.0	0.0
Less: Other	0.0		0.0	0.0
Investment in transmission affiliates, balance at end of period	$ 39.5	$ 3.5	$ 39.5	$ 3.5

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