SEGMENT INFORMATION (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2021 USD ($) segment	Jun. 30, 2020 USD ($)
Segment Reporting [Abstract]
Number of reportable segments | segment			2
Segment reporting information
Operating revenues	$ 341.2	$ 312.8	$ 746.5	$ 682.2
Other operation and maintenance	96.4	99.3	191.8	191.1
Depreciation and amortization	46.6	43.2	92.6	86.2
Other income, net	10.4	8.4	19.0	16.7
Interest expense	16.3	16.0	32.5	32.3
Income tax expense	8.5	12.1	17.2	28.8
Net income	60.2	47.7	129.5	115.9
Utility
Segment reporting information
Operating revenues	341.2	312.8	746.5	682.2
Other operation and maintenance	96.4	99.3	191.8	191.1
Depreciation and amortization	46.6	43.2	92.6	86.2
Other income, net	9.9	8.0	18.1	15.9
Interest expense	16.3	16.0	32.5	32.3
Income tax expense	8.4	12.0	17.0	28.6
Net income	59.8	47.4	128.8	115.3
Other
Segment reporting information
Operating revenues	0.0	0.0	0.0	0.0
Other operation and maintenance	0.0	0.0	0.0	0.0
Depreciation and amortization	0.0	0.0	0.0	0.0
Other income, net	0.5	0.4	0.9	0.8
Interest expense	0.0	0.0	0.0	0.0
Income tax expense	0.1	0.1	0.2	0.2
Net income	$ 0.4	$ 0.3	$ 0.7	$ 0.6