v3.26.1
Investments Fair Value (Details) - USD ($)
$ in Millions
Jun. 30, 2026
Dec. 31, 2025
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Equity securities $ 2,679.0 $ 2,487.7
Available for sale, Fixed income securities 12,161.2 12,709.8
Short-term investments 2,233.1 1,613.6
Fair Value, Inputs, Level 1 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Short-term investments 2,233.1 1,613.6
Fair Value, Inputs, Level 2 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Short-term investments 0.0 0.0
Fair Value, Inputs, Level 3 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Short-term investments 0.0 0.0
U.S. & Canadian Governments [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available for sale, Fixed income securities 1,665.0 1,787.4
U.S. & Canadian Governments [Member] | Fair Value, Inputs, Level 1 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available for sale, Fixed income securities 1,502.5 1,591.8
U.S. & Canadian Governments [Member] | Fair Value, Inputs, Level 2 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available for sale, Fixed income securities 162.4 195.6
U.S. & Canadian Governments [Member] | Fair Value, Inputs, Level 3 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available for sale, Fixed income securities 0.0 0.0
Corporate [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available for sale, Fixed income securities 10,496.2 10,922.3
Corporate [Member] | Fair Value, Inputs, Level 1 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available for sale, Fixed income securities 0.0 0.0
Corporate [Member] | Fair Value, Inputs, Level 2 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available for sale, Fixed income securities 10,486.6 10,912.7
Corporate [Member] | Fair Value, Inputs, Level 3 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available for sale, Fixed income securities 9.6 9.5
Equity Securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Equity securities 2,679.0 2,487.7
Equity Securities [Member] | Fair Value, Inputs, Level 1 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Equity securities 2,676.8 2,485.5
Equity Securities [Member] | Fair Value, Inputs, Level 2 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Equity securities 0.0 0.0
Equity Securities [Member] | Fair Value, Inputs, Level 3 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Equity securities $ 2.2 $ 2.2