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OPERATING REVENUES - DISAGGREGATION OF NATURAL GAS UTILITY OPERATING REVENUES BY CUSTOMER CLASS (Details) - Revenues from contracts with customers - Utility segment - Transferred over time - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2026
Sep. 30, 2025
Sep. 30, 2026
Sep. 30, 2025
Disaggregation of Operating Revenues        
Revenues from contracts with customers $ 1,324.3 $ 1,199.9 $ 3,681.0 $ 3,382.1
Natural gas        
Disaggregation of Operating Revenues        
Revenues from contracts with customers 48.8 53.2 426.0 378.8
Natural gas | Total retail        
Disaggregation of Operating Revenues        
Revenues from contracts with customers 39.3 37.3 396.6 360.3
Natural gas | Residential        
Disaggregation of Operating Revenues        
Revenues from contracts with customers 28.3 26.6 275.7 248.8
Natural gas | Commercial and industrial        
Disaggregation of Operating Revenues        
Revenues from contracts with customers 11.0 10.7 120.9 111.5
Natural gas | Transportation        
Disaggregation of Operating Revenues        
Revenues from contracts with customers 4.8 4.5 19.0 17.6
Natural gas | Other utility        
Disaggregation of Operating Revenues        
Revenues from contracts with customers $ 4.7 $ 11.4 $ 10.4 $ 0.9