SEGMENT INFORMATION (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2025 USD ($)	Jun. 30, 2024 USD ($)	Jun. 30, 2025 USD ($) numberOfSegments	Jun. 30, 2024 USD ($)	Dec. 31, 2024 USD ($)
Segment Reporting [Abstract]
Number of reportable segments | numberOfSegments			2
Segment information
External revenues	$ 410.5	$ 337.0	$ 909.2	$ 764.2
Fuel and purchased power	90.8	68.8	188.3	149.9
Cost of natural gas sold	32.7	20.9	125.4	94.7
Other operation and maintenance	121.8	111.3	241.1	215.9
Depreciation and amortization	66.7	59.2	132.5	118.3
Property and revenue taxes	11.0	11.5	22.4	23.5
Other income, net	4.2	12.5	8.5	24.0
Interest expense	24.2	23.9	48.2	47.9
Income tax expense	12.3	10.2	28.8	26.9
Net income	55.2	43.7	131.0	111.1
Capital expenditures	129.0	108.8	253.6	195.9
Equity method investments	16.8	15.0	16.8	15.0
Total assets	7,615.6	7,236.0	7,615.6	7,236.0	$ 7,495.7
Utility
Segment information
External revenues	410.5	337.0	909.2	764.2
Fuel and purchased power	90.8	68.8	188.3	149.9
Cost of natural gas sold	32.7	20.9	125.4	94.7
Other operation and maintenance	121.8	111.3	241.1	215.9
Depreciation and amortization	66.7	59.2	132.5	118.3
Property and revenue taxes	11.0	11.5	22.4	23.5
Other income, net	3.7	11.9	7.5	22.8
Interest expense	24.2	23.9	48.2	47.9
Income tax expense	12.2	10.0	28.6	26.6
Net income	54.8	43.3	130.2	110.2
Capital expenditures	129.0	108.8	253.6	195.9
Equity method investments	0.9	0.9	0.9	0.9
Total assets	7,599.7	7,221.9	7,599.7	7,221.9
Other
Segment information
External revenues	0.0	0.0	0.0	0.0
Fuel and purchased power	0.0	0.0	0.0	0.0
Cost of natural gas sold	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
Property and revenue taxes	0.0	0.0	0.0	0.0
Other income, net	0.5	0.6	1.0	1.2
Interest expense	0.0	0.0	0.0	0.0
Income tax expense	0.1	0.2	0.2	0.3
Net income	0.4	0.4	0.8	0.9
Capital expenditures	0.0	0.0	0.0	0.0
Equity method investments	15.9	14.1	15.9	14.1
Total assets	$ 15.9	$ 14.1	$ 15.9	$ 14.1