v3.26.1
Fair Value of Financial Instruments - Financial Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($)
$ in Millions
Mar. 31, 2026
Dec. 31, 2025
Assets:    
Investments $ 16.4 $ 17.2
Energy Marketing Contracts Assets, Current 0.8 0.0
Liabilities:    
Energy Marketing Contract Liabilities, Current 0.2 0.8
Quoted Prices in Active Markets (Level 1) [Member]    
Assets:    
Assets, Fair Value Disclosure 15.6 16.3
Quoted Prices in Active Markets (Level 1) [Member] | Total Investments [Member]    
Assets:    
Investments 15.6  
Significant Other Observable Inputs (Level 2) [Member]    
Assets:    
Assets, Fair Value Disclosure 0.9 0.0
Significant Unobservable Inputs (Level 3) [Member]    
Assets:    
Assets, Fair Value Disclosure 0.8 0.9
Significant Unobservable Inputs (Level 3) [Member] | Investments in guaranteed income fund [Member]    
Assets:    
Investments 0.8 0.9
Significant Unobservable Inputs (Level 3) [Member] | Total Investments [Member]    
Assets:    
Investments 0.8  
Recurring [Member]    
Assets:    
Assets, Fair Value Disclosure 17.3 17.2
Liabilities:    
Energy Marketing Contract Liabilities, Current   1.4
Recurring [Member] | Investments in guaranteed income fund [Member]    
Assets:    
Investments 0.8 $ 0.9
Recurring [Member] | Total Investments [Member]    
Assets:    
Investments 16.4  
Recurring [Member] | Mark To Market Energy Assets incl. natural gas and swap agreements[Member]    
Assets:    
Derivative assets $ 0.9