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INVESTMENT IN TRANSMISSION AFFILIATES - CHANGES TO INVESTMENT (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
Changes to investment in Transmission Affiliates
Investment in Transmission Affiliates, balance at beginning of period	$ 1,553.4	$ 1,443.9
Add: Earnings (loss) from equity method investment	32.8	41.9
Add: Capital contributions	12.8	27.6
Less: Other	0.1	0.1
Investment in Transmission Affiliates, balance at end of period	$ 1,598.9	1,513.3
ATC
Investment in Transmission Affiliates
Equity method investment, ownership interest (as a percent)	60.00%
Changes to investment in Transmission Affiliates
Investment in Transmission Affiliates, balance at beginning of period	$ 1,515.8	1,443.9
Add: Earnings (loss) from equity method investment	33.4	47.7
Add: Capital contributions	12.0	24.1
Less: Other	0.1	0.1
Investment in Transmission Affiliates, balance at end of period	1,561.1	1,515.6
Dividends receivable from ATC	$ 0.0		$ 39.9	$ 35.2
ATC Holdco
Investment in Transmission Affiliates
Equity method investment, ownership interest (as a percent)	75.00%
Changes to investment in Transmission Affiliates
Investment in Transmission Affiliates, balance at beginning of period	$ 37.6	0.0
Add: Earnings (loss) from equity method investment	(0.6)	(5.8)
Add: Capital contributions	0.8	3.5
Less: Other	0.0	0.0
Investment in Transmission Affiliates, balance at end of period	$ 37.8	$ (2.3)