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Fair Value Disclosures - Fair Value for Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($)
$ in Millions
Mar. 31, 2026
Dec. 31, 2025
Financial Assets    
Fixed maturity securities, fair value $ 8,783.8 $ 8,577.7
Equity securities at fair value 197.3 207.1
Other investments 601.5 573.0
States, municipalities and political subdivisions    
Financial Assets    
Fixed maturity securities, fair value 94.8 96.1
Foreign governments    
Financial Assets    
Fixed maturity securities, fair value 615.0 593.9
Asset-backed    
Financial Assets    
Fixed maturity securities, fair value 850.1 845.7
Commercial mortgage-backed    
Financial Assets    
Fixed maturity securities, fair value 419.0 417.4
Residential mortgage-backed    
Financial Assets    
Fixed maturity securities, fair value 1,001.5 954.7
U.S. corporate    
Financial Assets    
Fixed maturity securities, fair value 3,970.5 3,877.6
Foreign corporate    
Financial Assets    
Fixed maturity securities, fair value 1,769.8 1,729.4
Recurring    
Financial Assets    
Short-term investments 278.0 336.3
Other investments 69.5 72.2
Cash equivalents 1,156.7 1,349.3
Other assets 5.0 6.4
Total financial assets 10,490.3 10,549.0
Financial Liabilities    
Other liabilities 93.6 85.0
Total financial liabilities 93.6 85.0
Recurring | Level 1    
Financial Assets    
Short-term investments 269.8 329.0
Other investments 69.5 72.2
Cash equivalents 1,141.9 1,335.5
Other assets 0.0 0.0
Total financial assets 1,499.7 1,754.8
Financial Liabilities    
Other liabilities 61.9 62.7
Total financial liabilities 61.9 62.7
Recurring | Level 2    
Financial Assets    
Short-term investments 8.2 7.3
Other investments 0.0 0.0
Cash equivalents 14.8 13.8
Other assets 0.0 0.0
Total financial assets 8,740.7 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 5.0 6.4
Total financial assets 249.9 231.2
Financial Liabilities    
Other liabilities 31.7 22.3
Total financial liabilities 31.7 22.3
Recurring | U.S. government and government agencies and authorities    
Financial Assets    
Fixed maturity securities, fair value 63.1 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 63.1 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 94.8 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 94.8 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 615.0 593.9
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 615.0 593.9
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 850.1 845.7
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 713.4 715.9
Recurring | Asset-backed | Level 3    
Financial Assets    
Fixed maturity securities, fair value 136.7 129.8
Recurring | Commercial mortgage-backed    
Financial Assets    
Fixed maturity securities, fair value 419.0 417.4
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 419.0 417.4
Recurring | Commercial mortgage-backed | Level 3    
Financial Assets    
Fixed maturity securities, fair value 0.0 0.0
Recurring | Residential mortgage-backed    
Financial Assets    
Fixed maturity securities, fair value 1,001.5 954.7
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 1,001.5 954.7
Recurring | Residential mortgage-backed | Level 3    
Financial Assets    
Fixed maturity securities, fair value 0.0 0.0
Recurring | U.S. corporate    
Financial Assets    
Fixed maturity securities, fair value 3,970.5 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 3,897.8 3,812.0
Recurring | U.S. corporate | Level 3    
Financial Assets    
Fixed maturity securities, fair value 72.7 65.6
Recurring | Foreign corporate    
Financial Assets    
Fixed maturity securities, fair value 1,769.8 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 1,756.3 1,721.5
Recurring | Foreign corporate | Level 3    
Financial Assets    
Fixed maturity securities, fair value 13.5 7.9
Recurring | Mutual funds    
Financial Assets    
Equity securities at fair value 37.9 37.3
Recurring | Mutual funds | Level 1    
Financial Assets    
Equity securities at fair value 16.3 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 21.6 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 157.2 167.8
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 156.8 167.5
Recurring | Non-redeemable preferred stocks | Level 3    
Financial Assets    
Equity securities at fair value $ 0.4 $ 0.3