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Investment Securities (Fair Value Measurement by Level) (Details)
$ in Millions
Sep. 30, 2019
USD ($)
Investment
Dec. 31, 2018
USD ($)
Investment
Sep. 30, 2018
USD ($)
Dec. 31, 2017
USD ($)
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments $ 2,569.4 $ 2,328.7 $ 2,365.1 $ 3,208.3
Level 1 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 703.7 842.6 897.4 890.4
Level 2 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments $ 1,092.0 $ 1,160.5 1,178.0 2,105.4
Common Stock        
Debt and Equity Securities, FV-NI [Line Items]        
Number of Investments | Investment 2 2    
Debt securities issued by corporations        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments $ 463.2 $ 510.5    
Mortgage-backed and asset-backed securities        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 197.2 133.5    
Mortgage-backed and asset-backed securities | Level 2 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 197.2 133.5    
Mortgage-backed and asset-backed securities | Level 3 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 0.0 0.0    
Short-term investments        
Debt and Equity Securities, FV-NI [Line Items]        
Short-term investments, at fair value 272.5 214.2    
Other long-term investments | Level 3 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 586.8 138.7 127.0 77.2
Hedge and private equity funds included in other long-term investments | NAV        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 186.9 186.9 162.7 135.3
Fair value measured on a recurring basis        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 2,841.9 2,542.9    
Fair value measured on a recurring basis | Level 1 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 964.7 1,047.0    
Fair value measured on a recurring basis | Level 2 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 1,103.5 1,170.3    
Fair value measured on a recurring basis | Level 3 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 586.8 138.7    
Fair value measured on a recurring basis | U.S. Government and agency obligations        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 197.6 153.2    
Fair value measured on a recurring basis | U.S. Government and agency obligations | Level 1 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 197.6 153.2    
Fair value measured on a recurring basis | U.S. Government and agency obligations | Level 2 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 0.0 0.0    
Fair value measured on a recurring basis | U.S. Government and agency obligations | Level 3 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 0.0 0.0    
Fair value measured on a recurring basis | Debt securities issued by corporations        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 463.2 510.5    
Fair value measured on a recurring basis | Debt securities issued by corporations | Level 1 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 0.0 0.0    
Fair value measured on a recurring basis | Debt securities issued by corporations | Level 2 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 463.2 510.5    
Fair value measured on a recurring basis | Debt securities issued by corporations | Level 3 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 0.0 0.0    
Fair value measured on a recurring basis | Debt securities issued by corporations | Financials        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 143.3 143.4    
Fair value measured on a recurring basis | Debt securities issued by corporations | Financials | Level 1 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 0.0 0.0    
Fair value measured on a recurring basis | Debt securities issued by corporations | Financials | Level 2 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 143.3 143.4    
Fair value measured on a recurring basis | Debt securities issued by corporations | Financials | Level 3 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments   0.0    
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 55.0 55.0    
Fair value measured on a recurring basis | Debt securities issued by corporations | Health Care | Level 1 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 0.0 0.0    
Fair value measured on a recurring basis | Debt securities issued by corporations | Health Care | Level 2 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 55.0 55.0    
Fair value measured on a recurring basis | Debt securities issued by corporations | Health Care | Level 3 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 0.0 0.0    
Fair value measured on a recurring basis | Debt securities issued by corporations | Industrial        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 60.5 47.6    
Fair value measured on a recurring basis | Debt securities issued by corporations | Industrial | Level 1 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 0.0 0.0    
Fair value measured on a recurring basis | Debt securities issued by corporations | Industrial | Level 2 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 60.5 47.6    
Fair value measured on a recurring basis | Debt securities issued by corporations | Industrial | Level 3 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 0.0 0.0    
Fair value measured on a recurring basis | Debt securities issued by corporations | Technology        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 44.5 60.5    
Fair value measured on a recurring basis | Debt securities issued by corporations | Technology | Level 1 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 0.0 0.0    
Fair value measured on a recurring basis | Debt securities issued by corporations | Technology | Level 2 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 44.5 60.5    
Fair value measured on a recurring basis | Debt securities issued by corporations | Technology | Level 3 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 0.0 0.0    
Fair value measured on a recurring basis | Debt securities issued by corporations | Energy        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 45.0 57.6    
Fair value measured on a recurring basis | Debt securities issued by corporations | Energy | Level 1 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 0.0 0.0    
Fair value measured on a recurring basis | Debt securities issued by corporations | Energy | Level 2 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 45.0 57.6    
Fair value measured on a recurring basis | Debt securities issued by corporations | Energy | Level 3 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 0.0 0.0    
Fair value measured on a recurring basis | Debt securities issued by corporations | Consumer        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 49.3 68.5    
Fair value measured on a recurring basis | Debt securities issued by corporations | Consumer | Level 1 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 0.0 0.0    
Fair value measured on a recurring basis | Debt securities issued by corporations | Consumer | Level 2 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 49.3 68.5    
Fair value measured on a recurring basis | Debt securities issued by corporations | Consumer | Level 3 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 0.0 0.0    
Fair value measured on a recurring basis | Debt securities issued by corporations | Communications        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 27.1 31.8    
Fair value measured on a recurring basis | Debt securities issued by corporations | Communications | Level 1 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 0.0 0.0    
Fair value measured on a recurring basis | Debt securities issued by corporations | Communications | Level 2 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 27.1 31.8    
Fair value measured on a recurring basis | Debt securities issued by corporations | Communications | Level 3 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 0.0 0.0    
Fair value measured on a recurring basis | Debt securities issued by corporations | Utilities        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 22.3 19.8    
Fair value measured on a recurring basis | Debt securities issued by corporations | Utilities | Level 1 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 0.0 0.0    
Fair value measured on a recurring basis | Debt securities issued by corporations | Utilities | Level 2 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 22.3 19.8    
Fair value measured on a recurring basis | Debt securities issued by corporations | Utilities | Level 3 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 0.0 0.0    
Fair value measured on a recurring basis | Debt securities issued by corporations | Materials        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 16.2 26.3    
Fair value measured on a recurring basis | Debt securities issued by corporations | Materials | Level 1 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 0.0 0.0    
Fair value measured on a recurring basis | Debt securities issued by corporations | Materials | Level 2 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 16.2 26.3    
Fair value measured on a recurring basis | Debt securities issued by corporations | Materials | Level 3 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 0.0 0.0    
Fair value measured on a recurring basis | Mortgage-backed and asset-backed securities        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 197.2 133.5    
Fair value measured on a recurring basis | Mortgage-backed and asset-backed securities | Level 1 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 0.0 0.0    
Fair value measured on a recurring basis | Mortgage-backed and asset-backed securities | Level 2 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 197.2 133.5    
Fair value measured on a recurring basis | Mortgage-backed and asset-backed securities | Level 3 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 0.0 0.0    
Fair value measured on a recurring basis | Municipal obligations        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 308.0 280.3    
Fair value measured on a recurring basis | Municipal obligations | Level 1 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 0.0 0.0    
Fair value measured on a recurring basis | Municipal obligations | Level 2 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 308.0 280.3    
Fair value measured on a recurring basis | Municipal obligations | Level 3 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 0.0 0.0    
Fair value measured on a recurring basis | Fixed maturity investments        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 1,166.0 1,077.5    
Fair value measured on a recurring basis | Fixed maturity investments | Level 1 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 197.6 153.2    
Fair value measured on a recurring basis | Fixed maturity investments | Level 2 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 968.4 924.3    
Fair value measured on a recurring basis | Fixed maturity investments | Level 3 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 0.0 0.0    
Fair value measured on a recurring basis | Short-term investments        
Debt and Equity Securities, FV-NI [Line Items]        
Short-term investments, at fair value 272.5 214.2    
Fair value measured on a recurring basis | Short-term investments | Level 1 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Short-term investments, at fair value 261.0 204.4 $ 313.7 $ 176.1
Fair value measured on a recurring basis | Short-term investments | Level 2 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Short-term investments, at fair value 11.5 9.8    
Fair value measured on a recurring basis | Short-term investments | Level 3 Inputs        
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 | Common equity securities | Level 1 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 506.1      
Fair value measured on a recurring basis | Common equity securities | Level 2 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 123.6      
Fair value measured on a recurring basis | Common equity securities | Financials        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments   13.5    
Fair value measured on a recurring basis | Common equity securities | Financials | Level 1 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments   13.5    
Fair value measured on a recurring basis | Common equity securities | Financials | Level 2 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments   0.0    
Fair value measured on a recurring basis | Common equity securities | Financials | Level 3 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments   0.0    
Fair value measured on a recurring basis | Common equity securities | Health Care        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments   14.0    
Fair value measured on a recurring basis | Common equity securities | Health Care | Level 1 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments   14.0    
Fair value measured on a recurring basis | Common equity securities | Health Care | Level 2 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments   0.0    
Fair value measured on a recurring basis | Common equity securities | Health Care | Level 3 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments   0.0    
Fair value measured on a recurring basis | Common equity securities | Industrial        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments   11.4    
Fair value measured on a recurring basis | Common equity securities | Industrial | Level 1 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments   11.4    
Fair value measured on a recurring basis | Common equity securities | Industrial | Level 2 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments   0.0    
Fair value measured on a recurring basis | Common equity securities | Industrial | Level 3 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments   0.0    
Fair value measured on a recurring basis | Common equity securities | Technology        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments   7.4    
Fair value measured on a recurring basis | Common equity securities | Technology | Level 1 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments   7.4    
Fair value measured on a recurring basis | Common equity securities | Technology | Level 2 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments   0.0    
Fair value measured on a recurring basis | Common equity securities | Technology | Level 3 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments   0.0    
Fair value measured on a recurring basis | Common equity securities | Energy        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments   4.1    
Fair value measured on a recurring basis | Common equity securities | Energy | Level 1 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments   4.1    
Fair value measured on a recurring basis | Common equity securities | Energy | Level 2 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments   0.0    
Fair value measured on a recurring basis | Common equity securities | Energy | Level 3 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments   0.0    
Fair value measured on a recurring basis | Common equity securities | Consumer        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments   6.2    
Fair value measured on a recurring basis | Common equity securities | Consumer | Level 1 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments   6.2    
Fair value measured on a recurring basis | Common equity securities | Consumer | Level 2 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments   0.0    
Fair value measured on a recurring basis | Common equity securities | Consumer | Level 3 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments   0.0    
Fair value measured on a recurring basis | Common equity securities | Communications        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments   12.7    
Fair value measured on a recurring basis | Common equity securities | Communications | Level 1 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments   12.7    
Fair value measured on a recurring basis | Common equity securities | Communications | Level 2 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments   0.0    
Fair value measured on a recurring basis | Common equity securities | Communications | Level 3 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments   0.0    
Fair value measured on a recurring basis | Common equity securities | Materials        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments   3.1    
Fair value measured on a recurring basis | Common equity securities | Materials | Level 1 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments   3.1    
Fair value measured on a recurring basis | Common equity securities | Materials | Level 2 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments   0.0    
Fair value measured on a recurring basis | Common equity securities | Materials | Level 3 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments   0.0    
Fair value measured on a recurring basis | Common equity securities | Exchange traded funds        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 519.7 675.3    
Fair value measured on a recurring basis | Common equity securities | Exchange traded funds | Level 1 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 506.1 617.0    
Fair value measured on a recurring basis | Common equity securities | Exchange traded funds | Level 2 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 13.6 58.3    
Fair value measured on a recurring basis | Common equity securities | Exchange traded funds | Level 3 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 0.0 0.0    
Fair value measured on a recurring basis | Common equity securities | Other        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 110.0 177.9    
Fair value measured on a recurring basis | Common equity securities | Other | Level 1 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 0.0 0.0    
Fair value measured on a recurring basis | Common equity securities | Other | Level 2 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 110.0 177.9    
Fair value measured on a recurring basis | Common equity securities | Other | Level 3 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 0.0 0.0    
Fair value measured on a recurring basis | Common equity securities | Common equity securities        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 629.7 925.6    
Fair value measured on a recurring basis | Common equity securities | Common equity securities | Level 1 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments   689.4    
Fair value measured on a recurring basis | Common equity securities | Common equity securities | Level 2 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments   236.2    
Fair value measured on a recurring basis | Common equity securities | Common equity securities | Level 3 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 0.0 0.0    
Fair value measured on a recurring basis | Other long-term investments        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 586.8 138.7    
Fair value measured on a recurring basis | Other long-term investments | Level 1 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 0.0      
Fair value measured on a recurring basis | Other long-term investments | Level 2 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 0.0      
Fair value measured on a recurring basis | Other long-term investments | Level 3 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments $ 586.8 $ 138.7