v3.26.1
Fair Value Disclosures - Fair Value for Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($)
$ in Millions
Jun. 30, 2026
Dec. 31, 2025
Financial Assets    
Fixed maturity securities, fair value $ 8,989.5 $ 8,577.7
Equity securities at fair value 195.7 207.1
Other investments 631.7 573.0
States, municipalities and political subdivisions    
Financial Assets    
Fixed maturity securities, fair value 93.7 96.1
Foreign governments    
Financial Assets    
Fixed maturity securities, fair value 642.6 593.9
Asset-backed    
Financial Assets    
Fixed maturity securities, fair value 856.8 845.7
Commercial mortgage-backed    
Financial Assets    
Fixed maturity securities, fair value 467.0 417.4
Residential mortgage-backed    
Financial Assets    
Fixed maturity securities, fair value 1,081.2 954.7
U.S. corporate    
Financial Assets    
Fixed maturity securities, fair value 3,936.2 3,877.6
Foreign corporate    
Financial Assets    
Fixed maturity securities, fair value 1,849.2 1,729.4
Recurring    
Financial Assets    
Short-term investments 282.3 336.3
Other investments 78.6 72.2
Cash equivalents 1,230.0 1,349.3
Other assets 2.7 6.4
Total financial assets 10,778.8 10,549.0
Financial Liabilities    
Other liabilities 99.8 85.0
Total financial liabilities 99.8 85.0
Recurring | Level 1    
Financial Assets    
Short-term investments 257.1 329.0
Other investments 78.6 72.2
Cash equivalents 1,212.8 1,335.5
Other assets 0.0 0.0
Total financial assets 1,566.6 1,754.8
Financial Liabilities    
Other liabilities 73.1 62.7
Total financial liabilities 73.1 62.7
Recurring | Level 2    
Financial Assets    
Short-term investments 25.2 7.3
Other investments 0.0 0.0
Cash equivalents 17.2 13.8
Other assets 0.0 0.0
Total financial assets 8,903.7 8,563.0
Financial Liabilities    
Other liabilities 0.0 0.0
Total financial liabilities 0.0 0.0
Recurring | Level 3    
Financial Assets    
Short-term investments 0.0 0.0
Other investments 0.0 0.0
Cash equivalents 0.0 0.0
Other assets 2.7 6.4
Total financial assets 308.5 231.2
Financial Liabilities    
Other liabilities 26.7 22.3
Total financial liabilities 26.7 22.3
Recurring | U.S. government and government agencies and authorities    
Financial Assets    
Fixed maturity securities, fair value 62.8 62.9
Recurring | U.S. government and government agencies and authorities | Level 1    
Financial Assets    
Fixed maturity securities, fair value 0.0 0.0
Recurring | U.S. government and government agencies and authorities | Level 2    
Financial Assets    
Fixed maturity securities, fair value 62.8 62.9
Recurring | U.S. government and government agencies and authorities | Level 3    
Financial Assets    
Fixed maturity securities, fair value 0.0 0.0
Recurring | States, municipalities and political subdivisions    
Financial Assets    
Fixed maturity securities, fair value 93.7 96.1
Recurring | States, municipalities and political subdivisions | Level 1    
Financial Assets    
Fixed maturity securities, fair value 0.0 0.0
Recurring | States, municipalities and political subdivisions | Level 2    
Financial Assets    
Fixed maturity securities, fair value 93.7 96.1
Recurring | States, municipalities and political subdivisions | Level 3    
Financial Assets    
Fixed maturity securities, fair value 0.0 0.0
Recurring | Foreign governments    
Financial Assets    
Fixed maturity securities, fair value 642.6 593.9
Recurring | Foreign governments | Level 1    
Financial Assets    
Fixed maturity securities, fair value 0.0 0.0
Recurring | Foreign governments | Level 2    
Financial Assets    
Fixed maturity securities, fair value 642.6 593.9
Recurring | Foreign governments | Level 3    
Financial Assets    
Fixed maturity securities, fair value 0.0 0.0
Recurring | Asset-backed    
Financial Assets    
Fixed maturity securities, fair value 856.8 845.7
Recurring | Asset-backed | Level 1    
Financial Assets    
Fixed maturity securities, fair value 0.0 0.0
Recurring | Asset-backed | Level 2    
Financial Assets    
Fixed maturity securities, fair value 709.7 715.9
Recurring | Asset-backed | Level 3    
Financial Assets    
Fixed maturity securities, fair value 147.1 129.8
Recurring | Commercial mortgage-backed    
Financial Assets    
Fixed maturity securities, fair value 467.0 417.4
Recurring | Commercial mortgage-backed | Level 1    
Financial Assets    
Fixed maturity securities, fair value 0.0 0.0
Recurring | Commercial mortgage-backed | Level 2    
Financial Assets    
Fixed maturity securities, fair value 464.8 417.4
Recurring | Commercial mortgage-backed | Level 3    
Financial Assets    
Fixed maturity securities, fair value 2.2 0.0
Recurring | Residential mortgage-backed    
Financial Assets    
Fixed maturity securities, fair value 1,081.2 954.7
Recurring | Residential mortgage-backed | Level 1    
Financial Assets    
Fixed maturity securities, fair value 0.0 0.0
Recurring | Residential mortgage-backed | Level 2    
Financial Assets    
Fixed maturity securities, fair value 1,077.2 954.7
Recurring | Residential mortgage-backed | Level 3    
Financial Assets    
Fixed maturity securities, fair value 4.0 0.0
Recurring | U.S. corporate    
Financial Assets    
Fixed maturity securities, fair value 3,936.2 3,877.6
Recurring | U.S. corporate | Level 1    
Financial Assets    
Fixed maturity securities, fair value 0.0 0.0
Recurring | U.S. corporate | Level 2    
Financial Assets    
Fixed maturity securities, fair value 3,832.6 3,812.0
Recurring | U.S. corporate | Level 3    
Financial Assets    
Fixed maturity securities, fair value 103.6 65.6
Recurring | Foreign corporate    
Financial Assets    
Fixed maturity securities, fair value 1,849.2 1,729.4
Recurring | Foreign corporate | Level 1    
Financial Assets    
Fixed maturity securities, fair value 0.0 0.0
Recurring | Foreign corporate | Level 2    
Financial Assets    
Fixed maturity securities, fair value 1,823.0 1,721.5
Recurring | Foreign corporate | Level 3    
Financial Assets    
Fixed maturity securities, fair value 26.2 7.9
Recurring | Mutual funds    
Financial Assets    
Equity securities at fair value 38.1 37.3
Recurring | Mutual funds | Level 1    
Financial Assets    
Equity securities at fair value 15.9 16.1
Recurring | Mutual funds | Level 2    
Financial Assets    
Equity securities at fair value 0.0 0.0
Recurring | Mutual funds | Level 3    
Financial Assets    
Equity securities at fair value 22.2 21.2
Recurring | Common stocks    
Financial Assets    
Equity securities at fair value 2.2 2.0
Recurring | Common stocks | Level 1    
Financial Assets    
Equity securities at fair value 2.2 2.0
Recurring | Common stocks | Level 2    
Financial Assets    
Equity securities at fair value 0.0 0.0
Recurring | Common stocks | Level 3    
Financial Assets    
Equity securities at fair value 0.0 0.0
Recurring | Non-redeemable preferred stocks    
Financial Assets    
Equity securities at fair value 155.4 167.8
Recurring | Non-redeemable preferred stocks | Level 1    
Financial Assets    
Equity securities at fair value 0.0 0.0
Recurring | Non-redeemable preferred stocks | Level 2    
Financial Assets    
Equity securities at fair value 154.9 167.5
Recurring | Non-redeemable preferred stocks | Level 3    
Financial Assets    
Equity securities at fair value $ 0.5 $ 0.3