v3.26.1
Regulatory Matters: Regulatory Matters (Details) - USD ($)
$ in Millions
Jun. 30, 2026
Dec. 31, 2025
Regulatory Asset [Line Items]    
Regulatory assets $ 378.0 $ 394.7
Regulatory assets, current (121.0) (139.7)
Regulatory assets, non-current 257.0 255.0
Regulatory liabilities 588.7 588.2
Regulatory liabilities, current (94.4) (99.9)
Regulatory liabilities, non-current 494.3 488.3
Deferred energy and gas costs    
Regulatory Asset [Line Items]    
Regulatory liabilities 21.9 12.3
Employee benefit plan costs and related deferred taxes    
Regulatory Asset [Line Items]    
Regulatory liabilities 35.7 36.2
Cost of removal    
Regulatory Asset [Line Items]    
Regulatory liabilities 225.3 216.5
Excess deferred income taxes    
Regulatory Asset [Line Items]    
Regulatory liabilities 226.1 230.3
Colorado renewable energy    
Regulatory Asset [Line Items]    
Regulatory liabilities 38.3 33.2
Other regulatory liabilities    
Regulatory Asset [Line Items]    
Regulatory liabilities 9.7 8.2
Storm Costs    
Regulatory Asset [Line Items]    
Regulatory assets 18.1 50.7
Deferred energy and gas costs    
Regulatory Asset [Line Items]    
Regulatory assets 93.7 83.3
Deferred gas cost adjustments    
Regulatory Asset [Line Items]    
Regulatory assets 9.0 10.2
Gas price derivatives    
Regulatory Asset [Line Items]    
Regulatory assets 0.4 4.6
Deferred taxes on AFUDC    
Regulatory Asset [Line Items]    
Regulatory assets 11.2 10.6
Employee benefit plans and related deferred taxes    
Regulatory Asset [Line Items]    
Regulatory assets 85.9 87.4
Environmental    
Regulatory Asset [Line Items]    
Regulatory assets 13.2 13.1
Loss on reacquired debt    
Regulatory Asset [Line Items]    
Regulatory assets 13.3 14.1
Deferred taxes on flow through accounting    
Regulatory Asset [Line Items]    
Regulatory assets 98.1 94.8
Decommissioning costs    
Regulatory Asset [Line Items]    
Regulatory liabilities 31.7 51.5
Other regulatory assets    
Regulatory Asset [Line Items]    
Regulatory assets $ 35.1 $ 25.9