SEGMENT INFORMATION (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2016 USD ($) segment	Sep. 30, 2015 USD ($)
Segment Reporting [Abstract]
Number of reportable segments | segment			2
Segment reporting information
Revenues	$ 381.0	$ 390.8	$ 1,087.2	$ 1,146.1
Other operation and maintenance	110.1	120.3	359.3	356.0
Depreciation and amortization	30.7	30.4	91.7	90.5
Operating income (loss)	105.3	88.5	225.6	202.6
Other income, net	8.9	6.7	25.7	19.8
Interest expense	11.7	13.2	35.3	40.3
Intersegment revenues
Segment reporting information
Revenues	0.0	0.0	0.0	0.0
Reconciling Eliminations
Segment reporting information
Revenues	0.0	0.0	0.0	0.0
Other operation and maintenance	(0.1)	(0.6)	(0.4)	(0.6)
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)	(0.2)	(0.4)	(0.6)
Regulated Segment | Utility
Segment reporting information
Revenues	381.0	390.8	1,087.2	1,146.1
Other operation and maintenance	110.0	120.8	359.5	356.4
Depreciation and amortization	30.7	30.3	91.7	90.2
Operating income (loss)	105.5	88.4	225.5	202.6
Other income, net	6.3	4.3	18.5	11.1
Interest expense	11.7	13.1	35.2	40.1
Regulated Segment | Utility | Intersegment revenues
Segment reporting information
Revenues	0.0	0.0	0.0	0.0
Unregulated Segment | Other
Segment reporting information
Revenues	0.0	0.0	0.0	0.0
Other operation and maintenance	0.2	0.1	0.2	0.2
Depreciation and amortization	0.0	0.1	0.0	0.3
Operating income (loss)	(0.2)	0.1	0.1	0.0
Other income, net	2.6	2.4	7.2	8.7
Interest expense	0.0	0.1	0.1	0.2
Unregulated Segment | Other | Intersegment revenues
Segment reporting information
Revenues	$ 0.1	$ 0.2	$ 0.4	$ 0.6