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Investment Securities - Fair Value Measurement by Level (Details)
$ in Millions
Sep. 30, 2020
USD ($)
Investment
Dec. 31, 2019
USD ($)
Investment
Sep. 30, 2019
USD ($)
Dec. 31, 2018
USD ($)
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments $ 2,533.4 $ 2,746.0 $ 2,569.4 $ 2,328.7
Debt securities issued by corporations        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 528.7 467.2    
Mortgage and asset-backed securities        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 210.9 209.0    
Short-term investments        
Debt and Equity Securities, FV-NI [Line Items]        
Short-term investments, at fair value $ 571.8 $ 201.2 272.5 214.2
Common equity securities        
Debt and Equity Securities, FV-NI [Line Items]        
Number of investments | Investment 1 2    
Level 1        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments $ 295.3 $ 780.0 703.7 842.6
Level 2        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 1,066.8 1,109.6 1,092.0 1,160.5
Level 2 | Mortgage and asset-backed securities        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 210.9 209.0    
Level 3 | Mortgage and asset-backed securities        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 0.0 0.0    
Level 3 | Common equity securities        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 0.1 0.1    
Level 3 | Other long-term investments        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 1,000.3 654.0 $ 586.8 $ 138.7
Fair value measured on a recurring basis        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 3,105.2 2,947.2    
Fair value measured on a recurring basis | U.S. Government and agency obligations        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 210.6 232.5    
Fair value measured on a recurring basis | Debt securities issued by corporations        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 528.7 467.2    
Fair value measured on a recurring basis | Debt securities issued by corporations | Financials        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 142.9 144.8    
Fair value measured on a recurring basis | Debt securities issued by corporations | Industrial        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 61.5 59.0    
Fair value measured on a recurring basis | Debt securities issued by corporations | Technology        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 68.5 41.2    
Fair value measured on a recurring basis | Debt securities issued by corporations | Consumer        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 60.7 50.9    
Fair value measured on a recurring basis | Debt securities issued by corporations | Health Care        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 49.3 52.6    
Fair value measured on a recurring basis | Debt securities issued by corporations | Communications        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 42.3 31.3    
Fair value measured on a recurring basis | Debt securities issued by corporations | Energy        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 38.9 44.9    
Fair value measured on a recurring basis | Debt securities issued by corporations | Materials        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 37.4 17.5    
Fair value measured on a recurring basis | Debt securities issued by corporations | Utilities        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 27.2 25.0    
Fair value measured on a recurring basis | Municipal obligations        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 271.3 297.1    
Fair value measured on a recurring basis | Mortgage and asset-backed securities        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 210.9 209.0    
Fair value measured on a recurring basis | Fixed maturity investments        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 1,221.5 1,205.8    
Fair value measured on a recurring basis | Short-term investments        
Debt and Equity Securities, FV-NI [Line Items]        
Short-term investments, at fair value 571.8 201.2    
Fair value measured on a recurring basis | Common equity securities        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 140.7 683.9    
Fair value measured on a recurring basis | Common equity securities | Exchange traded funds        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 84.7 536.4    
Fair value measured on a recurring basis | Common equity securities | Other        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 56.0 147.5    
Fair value measured on a recurring basis | Other long-term investments        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 1,000.3 654.0    
Fair value measured on a recurring basis | Other long-term investments — NAV        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 170.9 202.3    
Fair value measured on a recurring basis | Level 1        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 865.9 969.4    
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 210.6 232.5    
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 | 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 | 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 | 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 | Health Care        
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 | 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 | 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 | Fixed maturity investments        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 210.6 232.5    
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 570.6 189.4    
Fair value measured on a recurring basis | Level 1 | Common equity securities        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 84.7 547.5    
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 84.7 521.6    
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 25.9    
Fair value measured on a recurring basis | Level 1 | Other long-term investments        
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 — NAV        
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 and Equity Securities, FV-NI [Line Items]        
Fair value investments 1,068.0 1,121.4    
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 528.7 467.2    
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 142.9 144.8    
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 61.5 59.0    
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 68.5 41.2    
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 60.7 50.9    
Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations | Health Care        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 49.3 52.6    
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 42.3 31.3    
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 38.9 44.9    
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 37.4 17.5    
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 27.2 25.0    
Fair value measured on a recurring basis | Level 2 | Municipal obligations        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 271.3 297.1    
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 210.9 209.0    
Fair value measured on a recurring basis | Level 2 | Fixed maturity investments        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 1,010.9 973.3    
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 1.2 11.8    
Fair value measured on a recurring basis | Level 2 | Common equity securities        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 55.9 136.3    
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 14.8    
Fair value measured on a recurring basis | Level 2 | Common equity securities | Other        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 55.9 121.5    
Fair value measured on a recurring basis | Level 2 | Other long-term investments        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 0.0 0.0    
Fair value measured on a recurring basis | Level 2 | Other long-term investments — NAV        
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 and Equity Securities, FV-NI [Line Items]        
Fair value investments 1,000.4 654.1    
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 | 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 | 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 | 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 | Health Care        
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 | 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 | 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 | 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 0.1 0.1    
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 0.1 0.1    
Fair value measured on a recurring basis | Level 3 | Other long-term investments        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 1,000.3 654.0    
Fair value measured on a recurring basis | Level 3 | Other long-term investments — NAV        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments $ 0.0 $ 0.0