SEGMENT INFORMATION (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) segment	Sep. 30, 2017 USD ($)
Segment Reporting [Abstract]
Number of reportable segments | segment			2
Segment reporting information
Operating revenues	$ 379.2	$ 380.7	$ 1,122.9	$ 1,111.9
Other operation and maintenance	114.6	102.4	322.7	318.4
Depreciation and amortization	35.7	35.1	105.2	103.9
Operating income (loss)	86.4	108.2	228.0	239.3
Other income, net	8.9	5.5	27.0	17.1
Interest expense	13.4	13.7	39.4	41.2
Regulated Segment | Utility
Segment reporting information
Operating revenues	379.2	380.7	1,122.9	1,111.9
Other operation and maintenance	114.5	102.5	322.4	318.0
Depreciation and amortization	35.7	35.0	105.2	103.9
Operating income (loss)	86.6	108.2	228.5	239.7
Other income, net	8.7	5.4	25.5	15.2
Interest expense	13.4	13.7	39.4	41.2
Unregulated Segment | Other
Segment reporting information
Operating revenues	0.0	0.0	0.0	0.0
Other operation and maintenance	0.1	(0.1)	0.3	0.4
Depreciation and amortization	0.0	0.1	0.0	0.0
Operating income (loss)	(0.2)	0.0	(0.5)	(0.4)
Other income, net	0.2	0.1	1.5	1.9
Interest expense	$ 0.0	$ 0.0	$ 0.0	$ 0.0