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Investment Securities (Fair Value Measurement by Level) (Details)
$ in Millions
Dec. 31, 2017
USD ($)
Investment
Dec. 31, 2016
USD ($)
Investment
Dec. 31, 2015
USD ($)
Investment securities      
Fair value investments $ 3,208.3 $ 2,547.3 $ 1,543.5
Debt securities issued by corporations      
Investment securities      
Fair value investments 880.9 746.3  
Mortgage and asset-backed securities      
Investment securities      
Fair value investments $ 694.7 979.8  
Common Stock      
Investment securities      
Number of investments | Investment 2    
Hedge and private equity funds included in other long-term investments      
Investment securities      
Carrying value $ 125.3 82.6  
Level 1      
Investment securities      
Fair value investments 890.4 279.5 789.0
Level 2      
Investment securities      
Fair value investments 2,105.4 2,093.8 585.6
Level 2 | Mortgage and asset-backed securities      
Investment securities      
Fair value investments 694.7 979.8  
Level 3 Inputs | Mortgage and asset-backed securities      
Investment securities      
Fair value investments 0.0 0.0  
Level 3 Inputs | Fixed maturity investments      
Investment securities      
Fair value investments $ 0.0 $ 0.0 0.0
Number of investments | Investment 3 3  
Level 3 Inputs | Other long-term investments      
Investment securities      
Fair value investments $ 87.2 $ 91.4 103.6
Level 3 Inputs | Hedge and private equity funds included in other long-term investments      
Investment securities      
Fair value investments 125.3 82.6 65.3
Fair value measured on a recurring basis      
Investment securities      
Fair value investments 3,259.1 2,639.7  
Fair value measured on a recurring basis | U.S. Government and agency obligations      
Investment securities      
Fair value investments 296.5 111.0  
Fair value measured on a recurring basis | Debt securities issued by corporations      
Investment securities      
Fair value investments 880.9 746.3  
Fair value measured on a recurring basis | Debt securities issued by corporations | Consumer      
Investment securities      
Fair value investments 185.1 190.8  
Fair value measured on a recurring basis | Debt securities issued by corporations | Communications      
Investment securities      
Fair value investments 127.8 72.0  
Fair value measured on a recurring basis | Debt securities issued by corporations | Financials      
Investment securities      
Fair value investments 114.8 79.7  
Fair value measured on a recurring basis | Debt securities issued by corporations | Utilities      
Investment securities      
Fair value investments 108.9 140.8  
Fair value measured on a recurring basis | Debt securities issued by corporations | Materials      
Investment securities      
Fair value investments 95.5 65.0  
Fair value measured on a recurring basis | Debt securities issued by corporations | Health Care      
Investment securities      
Fair value investments 94.3 114.9  
Fair value measured on a recurring basis | Debt securities issued by corporations | Technology      
Investment securities      
Fair value investments 80.5 48.8  
Fair value measured on a recurring basis | Debt securities issued by corporations | Energy      
Investment securities      
Fair value investments 48.1 6.1  
Fair value measured on a recurring basis | Debt securities issued by corporations | Industrial      
Investment securities      
Fair value investments 25.9 28.2  
Fair value measured on a recurring basis | Mortgage and asset-backed securities      
Investment securities      
Fair value investments 694.7 979.8  
Fair value measured on a recurring basis | Municipal obligations      
Investment securities      
Fair value investments 254.9 238.5  
Fair value measured on a recurring basis | Foreign government, agency and provincial obligations      
Investment securities      
Fair value investments 2.7 12.1  
Fair value measured on a recurring basis | Fixed maturity investments      
Investment securities      
Fair value investments 2,129.7 2,087.7  
Fair value measured on a recurring basis | Short-term investments      
Investment securities      
Fair value investments 176.1 175.0 $ 142.0
Fair value measured on a recurring basis | Common equity securities | Consumer      
Investment securities      
Fair value investments 10.7 8.6  
Fair value measured on a recurring basis | Common equity securities | Communications      
Investment securities      
Fair value investments 10.9 7.0  
Fair value measured on a recurring basis | Common equity securities | Financials      
Investment securities      
Fair value investments 16.3 7.7  
Fair value measured on a recurring basis | Common equity securities | Health Care      
Investment securities      
Fair value investments 17.1 13.9  
Fair value measured on a recurring basis | Common equity securities | Technology      
Investment securities      
Fair value investments 15.1 7.3  
Fair value measured on a recurring basis | Common equity securities | Energy      
Investment securities      
Fair value investments 3.8 2.5  
Fair value measured on a recurring basis | Common equity securities | Industrial      
Investment securities      
Fair value investments 11.9 1.5  
Fair value measured on a recurring basis | Common equity securities | Other      
Investment securities      
Fair value investments 210.6 79.9  
Fair value measured on a recurring basis | Common equity securities | Exchange Traded Funds      
Investment securities      
Fair value investments 569.7 157.2  
Fair value measured on a recurring basis | Common equity securities | Common equity securities      
Investment securities      
Fair value investments 866.1 285.6  
Fair value measured on a recurring basis | Other long-term investments      
Investment securities      
Fair value investments 87.2 91.4  
Fair value measured on a recurring basis | Level 1      
Investment securities      
Fair value investments 1,041.4 441.7  
Fair value measured on a recurring basis | Level 1 | U.S. Government and agency obligations      
Investment securities      
Fair value investments 296.5 101.5  
Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations      
Investment securities      
Fair value investments 0.0 0.0  
Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations | Consumer      
Investment securities      
Fair value investments 0.0 0.0  
Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations | Communications      
Investment securities      
Fair value investments 0.0 0.0  
Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations | Financials      
Investment securities      
Fair value investments 0.0 0.0  
Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations | Utilities      
Investment securities      
Fair value investments 0.0 0.0  
Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations | Materials      
Investment securities      
Fair value investments 0.0 0.0  
Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations | Health Care      
Investment securities      
Fair value investments 0.0 0.0  
Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations | Technology      
Investment securities      
Fair value investments 0.0 0.0  
Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations | Energy      
Investment securities      
Fair value investments 0.0 0.0  
Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations | Industrial      
Investment securities      
Fair value investments 0.0 0.0  
Fair value measured on a recurring basis | Level 1 | Mortgage and asset-backed securities      
Investment securities      
Fair value investments 0.0 0.0  
Fair value measured on a recurring basis | Level 1 | Municipal obligations      
Investment securities      
Fair value investments 0.0 0.0  
Fair value measured on a recurring basis | Level 1 | Foreign government, agency and provincial obligations      
Investment securities      
Fair value investments 0.0 0.0  
Fair value measured on a recurring basis | Level 1 | Fixed maturity investments      
Investment securities      
Fair value investments 296.5 101.5  
Fair value measured on a recurring basis | Level 1 | Short-term investments      
Investment securities      
Fair value investments 151.0 162.3  
Fair value measured on a recurring basis | Level 1 | Common equity securities | Consumer      
Investment securities      
Fair value investments 10.7 8.6  
Fair value measured on a recurring basis | Level 1 | Common equity securities | Communications      
Investment securities      
Fair value investments 10.9 7.0  
Fair value measured on a recurring basis | Level 1 | Common equity securities | Financials      
Investment securities      
Fair value investments 16.3 7.7  
Fair value measured on a recurring basis | Level 1 | Common equity securities | Health Care      
Investment securities      
Fair value investments 17.1 13.9  
Fair value measured on a recurring basis | Level 1 | Common equity securities | Technology      
Investment securities      
Fair value investments 15.1 7.3  
Fair value measured on a recurring basis | Level 1 | Common equity securities | Energy      
Investment securities      
Fair value investments 3.8 2.5  
Fair value measured on a recurring basis | Level 1 | Common equity securities | Industrial      
Investment securities      
Fair value investments 11.9 1.5  
Fair value measured on a recurring basis | Level 1 | Common equity securities | Other      
Investment securities      
Fair value investments 0.0 0.0  
Fair value measured on a recurring basis | Level 1 | Common equity securities | Exchange Traded Funds      
Investment securities      
Fair value investments 508.1 129.4  
Fair value measured on a recurring basis | Level 1 | Common equity securities | Common equity securities      
Investment securities      
Fair value investments 593.9 177.9  
Fair value measured on a recurring basis | Level 1 | Other long-term investments      
Investment securities      
Fair value investments 0.0 0.0  
Fair value measured on a recurring basis | Level 2      
Investment securities      
Fair value investments 2,130.5 2,106.6  
Fair value measured on a recurring basis | Level 2 | U.S. Government and agency obligations      
Investment securities      
Fair value investments 0.0 9.5  
Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations      
Investment securities      
Fair value investments 880.9 746.3  
Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations | Consumer      
Investment securities      
Fair value investments 185.1 190.8  
Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations | Communications      
Investment securities      
Fair value investments 127.8 72.0  
Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations | Financials      
Investment securities      
Fair value investments 114.8 79.7  
Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations | Utilities      
Investment securities      
Fair value investments 108.9 140.8  
Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations | Materials      
Investment securities      
Fair value investments 95.5 65.0  
Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations | Health Care      
Investment securities      
Fair value investments 94.3 114.9  
Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations | Technology      
Investment securities      
Fair value investments 80.5 48.8  
Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations | Energy      
Investment securities      
Fair value investments 48.1 6.1  
Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations | Industrial      
Investment securities      
Fair value investments 25.9 28.2  
Fair value measured on a recurring basis | Level 2 | Mortgage and asset-backed securities      
Investment securities      
Fair value investments 694.7 979.8  
Fair value measured on a recurring basis | Level 2 | Municipal obligations      
Investment securities      
Fair value investments 254.9 238.5  
Fair value measured on a recurring basis | Level 2 | Foreign government, agency and provincial obligations      
Investment securities      
Fair value investments 2.7 12.1  
Fair value measured on a recurring basis | Level 2 | Fixed maturity investments      
Investment securities      
Fair value investments 1,833.2 1,986.2  
Fair value measured on a recurring basis | Level 2 | Short-term investments      
Investment securities      
Fair value investments 25.1 12.7  
Fair value measured on a recurring basis | Level 2 | Common equity securities | Consumer      
Investment securities      
Fair value investments 0.0 0.0  
Fair value measured on a recurring basis | Level 2 | Common equity securities | Communications      
Investment securities      
Fair value investments 0.0 0.0  
Fair value measured on a recurring basis | Level 2 | Common equity securities | Financials      
Investment securities      
Fair value investments 0.0 0.0  
Fair value measured on a recurring basis | Level 2 | Common equity securities | Health Care      
Investment securities      
Fair value investments 0.0 0.0  
Fair value measured on a recurring basis | Level 2 | Common equity securities | Technology      
Investment securities      
Fair value investments 0.0 0.0  
Fair value measured on a recurring basis | Level 2 | Common equity securities | Energy      
Investment securities      
Fair value investments 0.0 0.0  
Fair value measured on a recurring basis | Level 2 | Common equity securities | Industrial      
Investment securities      
Fair value investments 0.0 0.0  
Fair value measured on a recurring basis | Level 2 | Common equity securities | Other      
Investment securities      
Fair value investments 210.6 79.9  
Fair value measured on a recurring basis | Level 2 | Common equity securities | Exchange Traded Funds      
Investment securities      
Fair value investments 61.6 27.8  
Fair value measured on a recurring basis | Level 2 | Common equity securities | Common equity securities      
Investment securities      
Fair value investments 272.2 107.7  
Fair value measured on a recurring basis | Level 2 | Other long-term investments      
Investment securities      
Fair value investments 0.0 0.0  
Fair value measured on a recurring basis | Level 3 Inputs      
Investment securities      
Fair value investments 87.2 91.4  
Fair value measured on a recurring basis | Level 3 Inputs | Foreign Exchange Forward [Member]      
Investment securities      
Fair value investments (3.7) (1.2)  
Fair value measured on a recurring basis | Level 3 Inputs | U.S. Government and agency obligations      
Investment securities      
Fair value investments 0.0 0.0  
Fair value measured on a recurring basis | Level 3 Inputs | Debt securities issued by corporations      
Investment securities      
Fair value investments 0.0 0.0  
Fair value measured on a recurring basis | Level 3 Inputs | Debt securities issued by corporations | Consumer      
Investment securities      
Fair value investments 0.0 0.0  
Fair value measured on a recurring basis | Level 3 Inputs | Debt securities issued by corporations | Communications      
Investment securities      
Fair value investments 0.0 0.0  
Fair value measured on a recurring basis | Level 3 Inputs | Debt securities issued by corporations | Financials      
Investment securities      
Fair value investments   0.0  
Fair value measured on a recurring basis | Level 3 Inputs | Debt securities issued by corporations | Utilities      
Investment securities      
Fair value investments 0.0 0.0  
Fair value measured on a recurring basis | Level 3 Inputs | Debt securities issued by corporations | Materials      
Investment securities      
Fair value investments 0.0 0.0  
Fair value measured on a recurring basis | Level 3 Inputs | Debt securities issued by corporations | Health Care      
Investment securities      
Fair value investments 0.0 0.0  
Fair value measured on a recurring basis | Level 3 Inputs | Debt securities issued by corporations | Technology      
Investment securities      
Fair value investments 0.0 0.0  
Fair value measured on a recurring basis | Level 3 Inputs | Debt securities issued by corporations | Energy      
Investment securities      
Fair value investments 0.0 0.0  
Fair value measured on a recurring basis | Level 3 Inputs | Debt securities issued by corporations | Industrial      
Investment securities      
Fair value investments 0.0 0.0  
Fair value measured on a recurring basis | Level 3 Inputs | Mortgage and asset-backed securities      
Investment securities      
Fair value investments 0.0 0.0  
Fair value measured on a recurring basis | Level 3 Inputs | Municipal obligations      
Investment securities      
Fair value investments 0.0 0.0  
Fair value measured on a recurring basis | Level 3 Inputs | Foreign government, agency and provincial obligations      
Investment securities      
Fair value investments 0.0 0.0  
Fair value measured on a recurring basis | Level 3 Inputs | Fixed maturity investments      
Investment securities      
Fair value investments 0.0 0.0  
Fair value measured on a recurring basis | Level 3 Inputs | Short-term investments      
Investment securities      
Fair value investments 0.0 0.0  
Fair value measured on a recurring basis | Level 3 Inputs | Common equity securities | Consumer      
Investment securities      
Fair value investments 0.0 0.0  
Fair value measured on a recurring basis | Level 3 Inputs | Common equity securities | Communications      
Investment securities      
Fair value investments 0.0 0.0  
Fair value measured on a recurring basis | Level 3 Inputs | Common equity securities | Financials      
Investment securities      
Fair value investments 0.0 0.0  
Fair value measured on a recurring basis | Level 3 Inputs | Common equity securities | Health Care      
Investment securities      
Fair value investments 0.0 0.0  
Fair value measured on a recurring basis | Level 3 Inputs | Common equity securities | Technology      
Investment securities      
Fair value investments 0.0 0.0  
Fair value measured on a recurring basis | Level 3 Inputs | Common equity securities | Energy      
Investment securities      
Fair value investments 0.0 0.0  
Fair value measured on a recurring basis | Level 3 Inputs | Common equity securities | Industrial      
Investment securities      
Fair value investments 0.0 0.0  
Fair value measured on a recurring basis | Level 3 Inputs | Common equity securities | Other      
Investment securities      
Fair value investments 0.0 0.0  
Fair value measured on a recurring basis | Level 3 Inputs | Common equity securities | Exchange Traded Funds      
Investment securities      
Fair value investments 0.0 0.0  
Fair value measured on a recurring basis | Level 3 Inputs | Common equity securities | Common equity securities      
Investment securities      
Fair value investments 0.0 0.0  
Fair value measured on a recurring basis | Level 3 Inputs | Other long-term investments      
Investment securities      
Fair value investments 87.2 91.4  
Forward Contracts | Fair value measured on a recurring basis | Level 3 Inputs | Other long-term investments      
Investment securities      
Carrying value $ (3.7) (1.2)  
Star & Shield Insurance Exchange | Fixed maturity investments      
Investment securities      
Fair value investments   6.6  
Star & Shield Insurance Exchange | Short-term investments      
Investment securities      
Fair value investments   $ 0.1