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Investment Securities - Fair Value Measurement by Level (Details) - USD ($)
$ in Millions
Jun. 30, 2023
Dec. 31, 2022
Jun. 30, 2022
Dec. 31, 2021
Debt and Equity Securities, FV-NI [Line Items]        
Other long-term investments $ 1,815.0 $ 1,488.0    
Debt securities issued by corporations        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 992.5 1,018.8    
Mortgage and asset-backed securities        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 288.0 254.2    
Collateralized loan obligations        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 187.8 182.9    
Level 2 | Mortgage and asset-backed securities        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 288.0 254.2    
Level 2 | Collateralized loan obligations        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 187.8 182.9    
Level 3 | Mortgage and asset-backed securities        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 0.0 0.0    
Level 3 | Collateralized loan obligations        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 0.0 0.0    
Level 3 | Other long-term investments        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 1,018.4 911.6 $ 933.3 $ 890.6
Fair value measured on a recurring basis        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 5,632.6 5,170.0    
Other long-term investments 1,815.0 1,488.0    
Fair value measured on a recurring basis | Marketing Technology (MediaAlpha)        
Debt and Equity Securities, FV-NI [Line Items]        
Other long-term investments 235.7 168.6    
Fair value measured on a recurring basis | U.S. Government and agency obligations        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 218.2 206.4    
Fair value measured on a recurring basis | Debt securities issued by corporations        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 992.5 1,018.8    
Fair value measured on a recurring basis | Debt securities issued by corporations | Financials        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 289.8 291.2    
Fair value measured on a recurring basis | Debt securities issued by corporations | Consumer        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 193.1 191.9    
Fair value measured on a recurring basis | Debt securities issued by corporations | Healthcare        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 122.7 121.3    
Fair value measured on a recurring basis | Debt securities issued by corporations | Technology        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 116.2 123.7    
Fair value measured on a recurring basis | Debt securities issued by corporations | Industrial        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 111.7 115.4    
Fair value measured on a recurring basis | Debt securities issued by corporations | Utilities        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 66.2 73.8    
Fair value measured on a recurring basis | Debt securities issued by corporations | Communications        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 42.7 47.9    
Fair value measured on a recurring basis | Debt securities issued by corporations | Energy        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 29.1 33.9    
Fair value measured on a recurring basis | Debt securities issued by corporations | Materials        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 21.0 19.7    
Fair value measured on a recurring basis | Municipal obligations        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 254.2 258.6    
Fair value measured on a recurring basis | Mortgage and asset-backed securities        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 288.0 254.2    
Fair value measured on a recurring basis | Collateralized loan obligations        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 187.8 182.9    
Fair value measured on a recurring basis | Fixed maturity investments        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 1,940.7 1,920.9    
Fair value measured on a recurring basis | Short-term investments        
Debt and Equity Securities, FV-NI [Line Items]        
Short-term investments, at fair value 986.1 924.1    
Fair value measured on a recurring basis | Common equity securities        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 655.1 668.4    
Fair value measured on a recurring basis | Common equity securities | Exchange traded funds        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 272.5 333.8    
Fair value measured on a recurring basis | Common equity securities | Other        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 382.6 334.6    
Fair value measured on a recurring basis | Other long-term investments        
Debt and Equity Securities, FV-NI [Line Items]        
Other long-term investments 1,033.3 926.4    
Fair value measured on a recurring basis | Other long-term investments — NAV        
Debt and Equity Securities, FV-NI [Line Items]        
Other long-term investments 781.7 561.6    
Fair value measured on a recurring basis | Level 1        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 1,708.6 1,632.9    
Other long-term investments 0.0 0.0    
Fair value measured on a recurring basis | Level 1 | Marketing Technology (MediaAlpha)        
Debt and Equity Securities, FV-NI [Line Items]        
Other long-term investments 235.7 168.6    
Fair value measured on a recurring basis | Level 1 | U.S. Government and agency obligations        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 218.2 206.4    
Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 0.0 0.0    
Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations | Financials        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 0.0 0.0    
Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations | Consumer        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 0.0 0.0    
Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations | Healthcare        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 0.0 0.0    
Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations | Technology        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 0.0 0.0    
Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations | Industrial        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 0.0 0.0    
Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations | Utilities        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 0.0 0.0    
Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations | Communications        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 0.0 0.0    
Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations | Energy        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 0.0 0.0    
Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations | Materials        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 0.0 0.0    
Fair value measured on a recurring basis | Level 1 | Municipal obligations        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 0.0 0.0    
Fair value measured on a recurring basis | Level 1 | Mortgage and asset-backed securities        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 0.0 0.0    
Fair value measured on a recurring basis | Level 1 | Collateralized loan obligations        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 0.0 0.0    
Fair value measured on a recurring basis | Level 1 | Fixed maturity investments        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 218.2 206.4    
Fair value measured on a recurring basis | Level 1 | Short-term investments        
Debt and Equity Securities, FV-NI [Line Items]        
Short-term investments, at fair value 982.2 924.1    
Fair value measured on a recurring basis | Level 1 | Common equity securities        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 272.5 333.8    
Fair value measured on a recurring basis | Level 1 | Common equity securities | Exchange traded funds        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 272.5 333.8    
Fair value measured on a recurring basis | Level 1 | Common equity securities | Other        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 0.0 0.0    
Fair value measured on a recurring basis | Level 1 | Other long-term investments        
Debt and Equity Securities, FV-NI [Line Items]        
Other long-term investments 0.0 0.0    
Fair value measured on a recurring basis | Level 1 | Other long-term investments — NAV        
Debt and Equity Securities, FV-NI [Line Items]        
Other long-term investments 0.0 0.0    
Fair value measured on a recurring basis | Level 2        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 2,123.9 2,063.9    
Other long-term investments 14.9 14.8    
Fair value measured on a recurring basis | Level 2 | Marketing Technology (MediaAlpha)        
Debt and Equity Securities, FV-NI [Line Items]        
Other long-term investments 0.0 0.0    
Fair value measured on a recurring basis | Level 2 | U.S. Government and agency obligations        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 0.0 0.0    
Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 992.5 1,018.8    
Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations | Financials        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 289.8 291.2    
Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations | Consumer        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 193.1 191.9    
Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations | Healthcare        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 122.7 121.3    
Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations | Technology        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 116.2 123.7    
Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations | Industrial        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 111.7 115.4    
Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations | Utilities        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 66.2 73.8    
Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations | Communications        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 42.7 47.9    
Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations | Energy        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 29.1 33.9    
Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations | Materials        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 21.0 19.7    
Fair value measured on a recurring basis | Level 2 | Municipal obligations        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 254.2 258.6    
Fair value measured on a recurring basis | Level 2 | Mortgage and asset-backed securities        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 288.0 254.2    
Fair value measured on a recurring basis | Level 2 | Collateralized loan obligations        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 187.8 182.9    
Fair value measured on a recurring basis | Level 2 | Fixed maturity investments        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 1,722.5 1,714.5    
Fair value measured on a recurring basis | Level 2 | Short-term investments        
Debt and Equity Securities, FV-NI [Line Items]        
Short-term investments, at fair value 3.9 0.0    
Fair value measured on a recurring basis | Level 2 | Common equity securities        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 382.6 334.6    
Fair value measured on a recurring basis | Level 2 | Common equity securities | Exchange traded funds        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 0.0 0.0    
Fair value measured on a recurring basis | Level 2 | Common equity securities | Other        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 382.6 334.6    
Fair value measured on a recurring basis | Level 2 | Other long-term investments        
Debt and Equity Securities, FV-NI [Line Items]        
Other long-term investments 14.9 14.8    
Fair value measured on a recurring basis | Level 2 | Other long-term investments — NAV        
Debt and Equity Securities, FV-NI [Line Items]        
Other long-term investments 0.0 0.0    
Fair value measured on a recurring basis | Level 3        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 1,018.4 911.6    
Other long-term investments 1,018.4 911.6    
Fair value measured on a recurring basis | Level 3 | Marketing Technology (MediaAlpha)        
Debt and Equity Securities, FV-NI [Line Items]        
Other long-term investments 0.0 0.0    
Fair value measured on a recurring basis | Level 3 | U.S. Government and agency obligations        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 0.0 0.0    
Fair value measured on a recurring basis | Level 3 | Debt securities issued by corporations        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 0.0 0.0    
Fair value measured on a recurring basis | Level 3 | Debt securities issued by corporations | Financials        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 0.0 0.0    
Fair value measured on a recurring basis | Level 3 | Debt securities issued by corporations | Consumer        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 0.0 0.0    
Fair value measured on a recurring basis | Level 3 | Debt securities issued by corporations | Healthcare        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 0.0 0.0    
Fair value measured on a recurring basis | Level 3 | Debt securities issued by corporations | Technology        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 0.0 0.0    
Fair value measured on a recurring basis | Level 3 | Debt securities issued by corporations | Industrial        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 0.0 0.0    
Fair value measured on a recurring basis | Level 3 | Debt securities issued by corporations | Utilities        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 0.0 0.0    
Fair value measured on a recurring basis | Level 3 | Debt securities issued by corporations | Communications        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 0.0 0.0    
Fair value measured on a recurring basis | Level 3 | Debt securities issued by corporations | Energy        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 0.0 0.0    
Fair value measured on a recurring basis | Level 3 | Debt securities issued by corporations | Materials        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 0.0 0.0    
Fair value measured on a recurring basis | Level 3 | Municipal obligations        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 0.0 0.0    
Fair value measured on a recurring basis | Level 3 | Mortgage and asset-backed securities        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 0.0 0.0    
Fair value measured on a recurring basis | Level 3 | Collateralized loan obligations        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 0.0 0.0    
Fair value measured on a recurring basis | Level 3 | Fixed maturity investments        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 0.0 0.0    
Fair value measured on a recurring basis | Level 3 | Short-term investments        
Debt and Equity Securities, FV-NI [Line Items]        
Short-term investments, at fair value 0.0 0.0    
Fair value measured on a recurring basis | Level 3 | Common equity securities        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 0.0 0.0    
Fair value measured on a recurring basis | Level 3 | Common equity securities | Exchange traded funds        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 0.0 0.0    
Fair value measured on a recurring basis | Level 3 | Common equity securities | Other        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 0.0 0.0    
Fair value measured on a recurring basis | Level 3 | Other long-term investments        
Debt and Equity Securities, FV-NI [Line Items]        
Other long-term investments 1,018.4 911.6    
Fair value measured on a recurring basis | Level 3 | Other long-term investments — NAV        
Debt and Equity Securities, FV-NI [Line Items]        
Other long-term investments $ 0.0 $ 0.0