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Investment Securities (Fair Value Measurement by Level) (Details)
$ in Millions
Mar. 31, 2019
USD ($)
Investment
Dec. 31, 2018
USD ($)
Investment
Mar. 31, 2018
USD ($)
Dec. 31, 2017
USD ($)
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments $ 2,534.6 $ 2,328.7 $ 2,532.5 $ 3,208.3
Level 1 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 799.3 842.6 874.3 890.4
Level 2 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments $ 1,237.7 $ 1,160.5 1,405.1 2,105.4
Common Stock        
Debt and Equity Securities, FV-NI [Line Items]        
Number of hedge funds | Investment 2 2    
Debt securities issued by corporations        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments $ 544.8 $ 510.5    
Mortgage-backed and asset-backed securities        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 148.8 133.5    
Mortgage-backed and asset-backed securities | Level 2 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 148.8 133.5    
Mortgage-backed and asset-backed securities | Level 3 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 0.0 0.0    
Other long-term investments | Level 3 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 275.1 138.7 113.4 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 222.5 186.9 139.7 135.3
Fair value measured on a recurring basis        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 2,804.4 2,542.9    
Fair value measured on a recurring basis | Level 1 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 1,048.3 1,047.0    
Fair value measured on a recurring basis | Level 2 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 1,258.5 1,170.3    
Fair value measured on a recurring basis | Level 3 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 275.1 138.7    
Fair value measured on a recurring basis | U.S. Government and agency obligations        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 153.0 153.2    
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 153.0 153.2    
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 544.8 510.5    
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 544.8 510.5    
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 139.7 143.4    
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 139.7 143.4    
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 74.7 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 74.7 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 | Energy        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 66.0 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 66.0 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 61.0 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 61.0 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 | Industrial        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 57.0 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 57.0 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 55.0 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 55.0 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 | Communications        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 40.4 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 40.4 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 28.5 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 28.5 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 22.5 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 22.5 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 148.8 133.5    
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 148.8 133.5    
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 285.4 280.3    
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 285.4 280.3    
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 1,132.0 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 153.0 153.2    
Fair value measured on a recurring basis | Fixed maturity investments | Level 2 Inputs        
Debt and Equity Securities, FV-NI [Line Items]        
Fair value investments 979.0 924.3    
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 269.8 214.2    
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 249.0 204.4 $ 763.0 $ 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 20.8 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 | 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 | 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 | 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 | 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 712.3 675.3    
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 646.3 617.0    
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 66.0 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 192.7 177.9    
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 192.7 177.9    
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 905.0 925.6    
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 646.3 689.4    
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 258.7 236.2    
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 275.1 138.7    
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 $ 275.1 $ 138.7