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Segment Information - Significant Segment Financial Data (Detail) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2025
Sep. 30, 2024
Sep. 30, 2025
Sep. 30, 2024
Dec. 31, 2024
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]          
Operation and Maintenance $ 21.9 $ 20.3 $ 65.8 $ 57.1  
Depreciation and Amortization 22.2 19.1 65.7 55.2  
Income Tax Expense (Benefit) (1.3) (1.3) 8.9 8.7  
Net Income Attributable to Common Shares (0.3)   31.0    
Segment Assets 1,944.7 1,737.1 1,944.7 1,737.1 $ 1,794.5
Capital Expenditures     127.4 114.3  
Operating Segments [Member] | Electric          
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]          
Total Operating Revenues 64.3 62.5 175.5 192.5  
Energy Supply Costs 31.2 32.8 89.1 110.8  
Operation and Maintenance 9.6 9.1 28.7 25.9  
Depreciation and Amortization 8.0 7.6 23.7 21.7  
Other Segment Expenses (Income) 4.0 3.4 9.4 9.7  
Interest Income (0.9) (2.5) (2.5) (4.2)  
Interest Expense 3.8 5.0 11.0 11.4  
Income Tax Expense (Benefit) 1.9 1.1 3.0 2.9  
Net Income Attributable to Common Shares 6.7 6.0 13.1 14.3  
Segment Assets 699.8 625.4 699.8 625.4  
Capital Expenditures 15.6 13.9 52.8 34.3  
Operating Segments [Member] | Gas Segment          
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]          
Total Operating Revenues 36.8 30.4 199.0 174.8  
Energy Supply Costs 10.2 7.1 64.3 59.2  
Operation and Maintenance 11.8 10.5 34.8 30.3  
Depreciation and Amortization 14.2 11.8 42.0 33.5  
Other Segment Expenses (Income) 4.4 3.8 13.1 13.1  
Interest Income (1.0) (1.3) (2.5) (3.1)  
Interest Expense 6.4 6.1 18.5 17.7  
Income Tax Expense (Benefit) (2.7) (2.4) 7.3 5.9  
Net Income Attributable to Common Shares (6.5) (5.2) 21.5 18.2  
Segment Assets 1,210.5 1,066.7 1,210.5 1,066.7  
Capital Expenditures 39.0 41.6 74.6 76.3  
Operating Segments [Member] | Total Reportable Segments [Member]          
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]          
Total Operating Revenues 101.1 92.9 374.5 367.3  
Energy Supply Costs 41.4 39.9 153.4 170.0  
Operation and Maintenance 21.4 19.6 63.5 56.2  
Depreciation and Amortization 22.2 19.4 65.7 55.2  
Other Segment Expenses (Income) 8.4 7.2 22.5 22.8  
Interest Income (1.9) (3.8) (5.0) (7.3)  
Interest Expense 10.2 11.1 29.5 29.1  
Income Tax Expense (Benefit) (0.8) (1.3) 10.3 8.8  
Net Income Attributable to Common Shares 0.2 0.8 34.6 32.5  
Segment Assets 1,910.3 1,692.1 1,910.3 1,692.1  
Capital Expenditures 54.6 55.5 127.4 110.6  
Operating Segments [Member] | All Other Segments          
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]          
Total Operating Revenues 0.0 0.0 0.0 0.0  
Energy Supply Costs 0.0 0.0 0.0 0.0  
Operation and Maintenance 0.5 0.7 2.3 0.9  
Depreciation and Amortization 0.0 0.3 0.0 0.0  
Other Segment Expenses (Income) (0.1) (0.3) (0.3) (0.1)  
Interest Income 0.1 0.9 0.1 0.0  
Interest Expense 0.5 0.8 2.7 0.3  
Income Tax Expense (Benefit) (0.5) 0.0 (1.4) (0.1)  
Net Income Attributable to Common Shares (0.5) (0.8) (3.4) (1.0)  
Segment Assets 34.4 45.0 34.4 45.0  
Capital Expenditures 0.0 1.9 0.0 3.7  
Operating Segments [Member] | Total [Member]          
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]          
Total Operating Revenues 101.1 92.9 374.5 367.3  
Energy Supply Costs 41.4 39.9 153.4 170.0  
Operation and Maintenance 21.9 20.3 65.8 57.1  
Depreciation and Amortization 22.2 19.1 65.7 55.2  
Other Segment Expenses (Income) 8.3 7.5 22.2 22.7  
Interest Income (1.8) (2.9) (4.9) (7.3)  
Interest Expense 10.7 10.3 32.2 29.4  
Income Tax Expense (Benefit) (1.3) (1.3) 8.9 8.7  
Net Income Attributable to Common Shares (0.3) 0.0 31.2 31.5  
Segment Assets 1,944.7 1,737.1 1,944.7 1,737.1  
Capital Expenditures $ 54.6 $ 57.4 $ 127.4 $ 114.3