v3.26.1
Segment Information - Significant Segment Financial Data (Detail) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 04, 2026
Jun. 30, 2026
Jun. 30, 2025
Jun. 30, 2026
Jun. 30, 2025
Dec. 31, 2025
Segment Reporting [Line Items]            
Operation and Maintenance   $ 23.8 $ 21.3 $ 47.2 $ 43.9  
Depreciation and Amortization   24.2 21.8 47.5 43.5  
Provision for Income Taxes $ 2.8 1.1 0.7 13.0 10.2  
Net Income Attributable to Common Shares   4.7   37.7    
Segment Assets   2,220.9 1,892.7 2,220.9 1,892.7 $ 2,134.2
Capital Expenditures       76.0 72.8  
Operating Segments [Member] | Electric            
Segment Reporting [Line Items]            
Total Operating Revenues   61.7 51.0 127.2 111.2  
Energy Supply Costs   30.1 25.2 66.0 57.9  
Operation and Maintenance   9.9 9.6 19.8 19.1  
Depreciation and Amortization   8.6 7.8 16.9 15.7  
Other Segment Expenses (Income)   4.0 2.2 7.6 5.4  
Interest Income   (0.7) (0.8) (1.3) (1.6)  
Interest Expense   3.9 3.8 8.0 7.2  
Provision for Income Taxes   1.3 0.4 2.2 1.1  
Net Income Attributable to Common Shares   4.6 2.8 8.0 6.4  
Segment Assets   763.9 677.3 763.9 677.3  
Capital Expenditures   18.9 16.0 39.6 37.2  
Operating Segments [Member] | Gas Segment            
Segment Reporting [Line Items]            
Total Operating Revenues   55.3 51.6 206.7 162.2  
Energy Supply Costs   14.7 14.4 84.0 54.1  
Operation and Maintenance   13.6 10.9 27.8 23.0  
Depreciation and Amortization   15.6 14.0 30.6 27.8  
Other Segment Expenses (Income)   4.0 3.9 9.5 8.7  
Interest Income   (0.7) (0.6) (1.5) (1.4)  
Interest Expense   6.5 5.9 13.4 12.0  
Provision for Income Taxes   0.2 0.6 11.4 10.0  
Net Income Attributable to Common Shares   1.4 2.5 31.5 28.0  
Segment Assets   1,368.0 1,185.5 1,368.0 1,185.5  
Capital Expenditures   24.7 24.2 36.4 35.6  
Operating Segments [Member] | Total Reportable Segments [Member]            
Segment Reporting [Line Items]            
Total Operating Revenues   117.0 102.6 333.9 273.4  
Energy Supply Costs   44.8 39.6 150.0 112.0  
Operation and Maintenance   23.5 20.5 47.6 42.1  
Depreciation and Amortization   24.2 21.8 47.5 43.5  
Other Segment Expenses (Income)   8.0 6.1 17.1 14.1  
Interest Income   (1.4) (1.4) (2.8) (3.0)  
Interest Expense   10.4 9.7 21.4 19.2  
Provision for Income Taxes   1.5 1.0 13.6 11.1  
Net Income Attributable to Common Shares   6.0 5.3 39.5 34.4  
Segment Assets   2,131.9 1,862.8 2,131.9 1,862.8  
Capital Expenditures   43.6 40.2 76.0 72.8  
Operating Segments [Member] | All Other Segments            
Segment Reporting [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   0.3 0.8 (0.4) 1.8  
Depreciation and Amortization   0.0 0.0 0.0 0.0  
Other Segment Expenses (Income)   0.0 (0.2) 0.0 (0.2)  
Interest Income   0.0 0.0 (0.1) 0.0  
Interest Expense   1.4 1.0 2.7 2.2  
Provision for Income Taxes   (0.4) (0.3) (0.6) (0.9)  
Net Income Attributable to Common Shares   (1.3) (1.3) (1.6) (2.9)  
Segment Assets   89.0 29.9 89.0 29.9  
Capital Expenditures   0.0 0.0 0.0 0.0  
Operating Segments [Member] | Total [Member]            
Segment Reporting [Line Items]            
Total Operating Revenues   117.0 102.6 333.9 273.4  
Energy Supply Costs   44.8 39.6 150.0 112.0  
Operation and Maintenance   23.8 21.3 47.2 43.9  
Depreciation and Amortization   24.2 21.8 47.5 43.5  
Other Segment Expenses (Income)   8.0 5.9 17.1 13.9  
Interest Income   (1.4) (1.4) (2.9) (3.0)  
Interest Expense   11.8 10.7 24.1 21.4  
Provision for Income Taxes   1.1 0.7 13.0 10.2  
Net Income Attributable to Common Shares   4.7 4.0 37.9 31.5  
Segment Assets   2,220.9 1,892.7 2,220.9 1,892.7  
Capital Expenditures   $ 43.6 $ 40.2 $ 76.0 $ 72.8