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Segment Information - Significant Segment Financial Data (Detail) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2024
Sep. 30, 2023
Sep. 30, 2024
Sep. 30, 2023
Dec. 31, 2023
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]          
Total Operating Revenues $ 92.9 $ 103.9 $ 367.3 $ 427.5  
Segment Profit (Loss) 0.0 1.4 31.5 29.7  
Capital Expenditures 57.4 35.8 114.3 93.4  
Segment Assets 1,737.1 1,620.2 1,737.1 1,620.2 $ 1,670.4
Billed and Unbilled Revenue          
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]          
Total Operating Revenues 92.8 105.0 369.9 439.8  
Rate Adjustment Mechanism Revenue          
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]          
Total Operating Revenues 0.1 (1.1) (2.6) (12.3)  
Gas Segment          
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]          
Total Operating Revenues 30.4 31.8 174.8 182.7  
Segment Profit (Loss) (5.2) (4.3) 18.2 15.6  
Capital Expenditures 41.6 28.3 76.3 61.8  
Segment Assets 1,066.7 989.2 1,066.7 989.2  
Gas Segment | Billed and Unbilled Revenue          
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]          
Total Operating Revenues 25.3 24.4 172.6 191.7  
Gas Segment | Rate Adjustment Mechanism Revenue          
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]          
Total Operating Revenues 5.1 7.4 2.2 (9.0)  
Electric          
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]          
Total Operating Revenues 62.5 72.1 192.5 244.8  
Segment Profit (Loss) 6.0 6.2 14.3 15.1  
Capital Expenditures 13.9 5.3 34.3 29.3  
Segment Assets 625.4 607.2 625.4 607.2  
Electric | Billed and Unbilled Revenue          
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]          
Total Operating Revenues 67.5 80.6 197.3 248.1  
Electric | Rate Adjustment Mechanism Revenue          
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]          
Total Operating Revenues (5.0) (8.5) (4.8) (3.3)  
All Other Segments          
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]          
Segment Profit (Loss) (0.8) (0.5) (1.0) (1.0)  
Capital Expenditures 1.9 2.2 3.7 2.3  
Segment Assets $ 45.0 $ 23.8 $ 45.0 $ 23.8