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Significant Segment Financial Data (Detail) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2023
Sep. 30, 2022
Sep. 30, 2023
Sep. 30, 2022
Dec. 31, 2022
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]          
Total Operating Revenues $ 103.9 $ 110.2 $ 427.5 $ 401.7  
Segment Profit (Loss) 1.4 0.5 29.7 26.9  
Capital Expenditures 35.8 37.2 93.4 82.5  
Segment Assets 1,620.2 1,578.9 1,620.2 1,578.9 $ 1,590.4
Billed and Unbilled Revenue          
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]          
Total Operating Revenues 105.0 105.2 439.8 417.3  
Rate Adjustment Mechanism Revenue          
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]          
Total Operating Revenues (1.1) 5.0 (12.3) (15.6)  
Gas Segment          
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]          
Total Operating Revenues 31.8 34.5 182.7 182.5  
Segment Profit (Loss) (4.3) (5.3) 15.6 14.5  
Capital Expenditures 28.3 28.9 61.8 59.9  
Segment Assets 989.2 967.5 989.2 967.5  
Gas Segment | Billed and Unbilled Revenue          
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]          
Total Operating Revenues 24.4 27.4 191.7 190.6  
Gas Segment | Rate Adjustment Mechanism Revenue          
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]          
Total Operating Revenues 7.4 7.1 (9.0) (8.1)  
Electric          
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]          
Total Operating Revenues 72.1 75.7 244.8 219.2  
Segment Profit (Loss) 6.2 5.9 15.1 13.2  
Capital Expenditures 5.3 8.3 29.3 22.5  
Segment Assets 607.2 590.9 607.2 590.9  
Electric | Billed and Unbilled Revenue          
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]          
Total Operating Revenues 80.6 77.8 248.1 226.7  
Electric | Rate Adjustment Mechanism Revenue          
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]          
Total Operating Revenues (8.5) (2.1) (3.3) (7.5)  
All Other Segments          
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]          
Segment Profit (Loss) (0.5) (0.1) (1.0) (0.8)  
Capital Expenditures 2.2   2.3 0.1  
Segment Assets $ 23.8 $ 20.5 $ 23.8 $ 20.5