SEGMENT INFORMATION (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($) segment	Sep. 30, 2020 USD ($)
Segment Reporting [Abstract]
Number of reportable segments | segment			2
Segment reporting information
Operating revenues	$ 381.1	$ 367.7	$ 1,127.6	$ 1,049.9
Other operation and maintenance	102.2	105.2	294.0	296.3
Depreciation and amortization	47.4	43.5	140.0	129.7
Other income, net	8.1	8.7	27.1	25.4
Interest expense	16.1	15.6	48.6	47.9
Income tax expense	10.4	19.6	27.6	48.4
Net income	71.9	76.3	201.4	192.2
Utility
Segment reporting information
Operating revenues	381.1	367.7	1,127.6	1,049.9
Other operation and maintenance	102.2	105.2	294.0	296.3
Depreciation and amortization	47.4	43.5	140.0	129.7
Other income, net	7.7	8.3	25.8	24.2
Interest expense	16.1	15.6	48.6	47.9
Income tax expense	10.3	19.5	27.3	48.1
Net income	71.6	76.0	200.4	191.3
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	0.4	0.4	1.3	1.2
Interest expense	0.0	0.0	0.0	0.0
Income tax expense	0.1	0.1	0.3	0.3
Net income	$ 0.3	$ 0.3	$ 1.0	$ 0.9