v3.25.2
Segment Information - Significant Segment Financial Data (Detail) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2025
Jun. 30, 2024
Jun. 30, 2025
Jun. 30, 2024
Dec. 31, 2024
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]          
Operation and Maintenance $ 21.3 $ 18.6 $ 43.9 $ 36.8  
Depreciation and Amortization 21.8 18.1 43.5 36.1  
Provision for Income Taxes 0.7 0.6 10.2 10.0  
Net Income Attributable to Common Shares 4.0   31.3    
Segment Assets 1,892.7 1,695.0 1,892.7 1,695.0 $ 1,794.5
Capital Expenditures     72.8 56.9  
Operating Segments [Member] | Electric          
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]          
Total Operating Revenues 51.0 56.4 111.2 130.0  
Energy Supply Costs 25.2 31.5 57.9 78.0  
Operation and Maintenance 9.6 8.4 19.1 16.8  
Depreciation and Amortization 7.8 7.1 15.7 14.1  
Other Segment Expenses (Income) 2.2 3.0 5.4 6.3  
Interest Income (0.8) (0.9) (1.6) (1.7)  
Interest Expense 3.8 3.3 7.2 6.4  
Provision for Income Taxes 0.4 0.6 1.1 1.8  
Net Income Attributable to Common Shares 2.8 3.4 6.4 8.3  
Segment Assets 677.3 632.9 677.3 632.9  
Capital Expenditures 16.0 11.7 37.2 20.4  
Operating Segments [Member] | Gas Segment          
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]          
Total Operating Revenues 51.6 39.3 162.2 144.4  
Energy Supply Costs 14.4 8.0 54.1 52.1  
Operation and Maintenance 10.9 10.0 23.0 19.8  
Depreciation and Amortization 14.0 10.8 27.8 21.7  
Other Segment Expenses (Income) 3.9 4.7 8.7 9.3  
Interest Income (0.6) (0.9) (1.4) (1.8)  
Interest Expense 5.9 5.6 12.0 11.6  
Provision for Income Taxes 0.6 0.1 10.0 8.3  
Net Income Attributable to Common Shares 2.5 1.0 28.0 23.4  
Segment Assets 1,185.5 1,035.3 1,185.5 1,035.3  
Capital Expenditures 24.2 24.0 35.6 34.7  
Operating Segments [Member] | Total Reportable Segments [Member]          
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]          
Total Operating Revenues 102.6 95.7 273.4 274.4  
Energy Supply Costs 39.6 39.5 112.0 130.1  
Operation and Maintenance 20.5 18.4 42.1 36.6  
Depreciation and Amortization 21.8 17.9 43.5 35.8  
Other Segment Expenses (Income) 6.1 7.7 14.1 15.6  
Interest Income (1.4) (1.8) (3.0) (3.5)  
Interest Expense 9.7 8.9 19.2 18.0  
Provision for Income Taxes 1.0 0.7 11.1 10.1  
Net Income Attributable to Common Shares 5.3 4.4 34.4 31.7  
Segment Assets 1,862.8 1,668.2 1,862.8 1,668.2  
Capital Expenditures 40.2 35.7 72.8 55.1  
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 0.8 0.2 1.8 0.2  
Depreciation and Amortization 0.0 0.2 0.0 0.3  
Other Segment Expenses (Income) (0.2) (0.5) (0.2) (0.4)  
Interest Income 0.0 (0.6) 0.0 (0.9)  
Interest Expense 1.0 0.9 2.2 1.1  
Provision for Income Taxes (0.3) (0.1) (0.9) (0.1)  
Net Income Attributable to Common Shares (1.3) (0.1) (2.9) (0.2)  
Segment Assets 29.9 26.8 29.9 26.8  
Capital Expenditures 0.0 1.0 0.0 1.8  
Operating Segments [Member] | Total [Member]          
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]          
Total Operating Revenues 102.6 95.7 273.4 274.4  
Energy Supply Costs 39.6 39.5 112.0 130.1  
Operation and Maintenance 21.3 18.6 43.9 36.8  
Depreciation and Amortization 21.8 18.1 43.5 36.1  
Other Segment Expenses (Income) 5.9 7.2 13.9 15.2  
Interest Income (1.4) (2.4) (3.0) (4.4)  
Interest Expense 10.7 9.8 21.4 19.1  
Provision for Income Taxes 0.7 0.6 10.2 10.0  
Net Income Attributable to Common Shares 4.0 4.3 31.5 31.5  
Segment Assets 1,892.7 1,695.0 1,892.7 1,695.0  
Capital Expenditures $ 40.2 $ 36.7 $ 72.8 $ 56.9