SEGMENTS INFORMATION (Details) $ in Millions	12 Months Ended
	Dec. 31, 2021 USD ($) segment	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Segment Reporting [Abstract]
Number of reportable segments | segment	2
Segment reporting information
External revenues	$ 1,520.9	$ 1,407.1	$ 1,411.9
Other operation and maintenance	406.4	426.3	396.2
Depreciation and amortization	188.6	174.3	166.2
Other income, net	38.2	31.4	39.6
Interest expense	64.7	63.5	63.4
Income tax expense	31.2	54.5	59.1
Net income	231.1	221.6	184.7
Capital expenditures	389.7	530.7	517.8
Total assets	6,235.7	5,933.9	5,641.6
Utility
Segment reporting information
External revenues	1,520.9	1,407.1	1,411.9
Other operation and maintenance	406.4	426.3	396.2
Depreciation and amortization	188.6	174.3	166.2
Other income, net	36.8	29.9	38.2
Interest expense	64.7	63.5	63.4
Income tax expense	30.9	54.2	58.7
Net income	230.0	220.4	183.7
Capital expenditures	389.7	530.7	517.8
Total assets	6,224.3	5,923.3	5,595.7
Other
Segment reporting information
External revenues	0.0	0.0	0.0
Other operation and maintenance	0.0	0.0	0.0
Depreciation and amortization	0.0	0.0	0.0
Other income, net	1.4	1.5	1.4
Interest expense	0.0	0.0	0.0
Income tax expense	0.3	0.3	0.4
Net income	1.1	1.2	1.0
Capital expenditures	0.0	0.0	0.0
Total assets	$ 11.4	$ 10.6	$ 45.9