INVESTMENT IN AMERICAN TRANSMISSION COMPANY - CHANGES TO INVESTMENT (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Changes to investment in ATC
Investment in ATC, balance at beginning of period			$ 1,443.9
Add: Earnings from equity method investment	$ 39.2	$ 38.3	122.9	$ 107.7
Investment in ATC, balance at end of period	$ 1,560.8		$ 1,560.8
ATC
Investment in ATC
Ownership interest in ATC (as a percent)	60.00%		60.00%
Changes to investment in ATC
Investment in ATC, balance at beginning of period	$ 1,544.0	1,425.0	$ 1,443.9	1,380.9
Add: Earnings from equity method investment	39.2	38.3	122.9	107.7
Add: Capital contributions	12.8	15.0	63.3	27.1
Acquisition of Integrys's investment in ATC	0.0	0.0	0.0	(1.0)
Add: Adjustment to equity method goodwill	0.0	0.0	0.0	10.4
Less: Distributions	35.2	25.2	69.2	71.9
Less: Other	0.0	0.0	0.1	0.1
Investment in ATC, balance at end of period	$ 1,560.8	$ 1,453.1	$ 1,560.8	$ 1,453.1
Dividends Receivable					$ 35.2

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

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