SEGMENT INFORMATION (Details) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($) segment	Jun. 30, 2016 USD ($)	Dec. 31, 2016
Segment Reporting [Abstract]
Number of reportable segments \| segment			2
Segment reporting information
Revenues	$ 341.6	$ 331.5	$ 731.2	$ 706.2
Other operation and maintenance	105.9	134.1	210.3	249.2
Depreciation and amortization	34.4	30.2	68.8	61.0
Operating income (loss)	61.0	43.4	136.8	120.3
Other income, net	3.0	8.9	5.9	16.8
Interest expense	13.6	11.7	27.5	23.6
Intersegment revenues
Segment reporting information
Revenues		0.0		0.0
Reconciling Eliminations
Segment reporting information
Revenues	0.0	0.0	0.0	0.0
Other operation and maintenance	0.0	(0.1)	0.0	(0.3)
Depreciation and amortization	0.0	0.0	0.0	0.0
Operating income (loss)	0.0	0.0	0.0	0.0
Other income, net	0.0	0.0	0.0	0.0
Interest expense	0.0	0.0	0.0	0.0
Reconciling Eliminations \| Intersegment revenues
Segment reporting information
Revenues		(0.1)		(0.3)
Regulated Segment \| Utility
Segment reporting information
Revenues	341.6	331.5	731.2	706.2
Other operation and maintenance	105.6	134.3	209.8	249.5
Depreciation and amortization	34.5	30.3	68.9	61.0
Operating income (loss)	61.2	43.1	137.2	120.0
Other income, net	1.6	6.8	4.1	12.2
Interest expense	13.6	11.7	27.5	23.5
Regulated Segment \| Utility \| Intersegment revenues
Segment reporting information
Revenues		0.0		0.0
Unregulated Segment \| Other
Segment reporting information
Revenues	0.0	0.0	0.0	0.0
Other operation and maintenance	0.3	(0.1)	0.5	0.0
Depreciation and amortization	(0.1)	(0.1)	(0.1)	0.0
Operating income (loss)	(0.2)	0.3	(0.4)	0.3
Other income, net	1.4	2.1	1.8	4.6
Interest expense	$ 0.0	0.0	$ 0.0	0.1
Unregulated Segment \| Other \| Intersegment revenues
Segment reporting information
Revenues		$ 0.1		$ 0.3
ATC
Segment reporting information
Equity method investment, ownership percentage	34.00%		34.00%
WPS Investments, LLC
Segment reporting information
Equity method investment, ownership percentage					10.37%

The aggregate expense recognized in the current period that allocates the cost of tangible assets, intangible assets, or depleting assets to periods that benefit from use of the assets.

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The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting.

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