INVESTMENT IN AMERICAN TRANSMISSION COMPANY - CHANGES TO INVESTMENT (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2017	Mar. 31, 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	41.9	$ 38.5
Investment in ATC, balance at end of period	$ 1,513.3
ATC
Investment in ATC
Ownership interest in ATC (as a percent)	60.00%
Changes to investment in ATC
Investment in ATC, balance at beginning of period	$ 1,443.9	1,380.9
Add: Earnings from equity method investment	41.9	38.5
Add: Capital contributions	27.6	9.0
Add: Adjustment to equity method goodwill	0.0	9.3
Less: Distributions	0.0	15.1
Less: Other	0.1	0.1
Investment in ATC, balance at end of period	$ 1,513.3	$ 1,422.5
Dividends Receivable			$ 35.2

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

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