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Investment Securities (Fair Value Measurement by Level) (Details) - USD ($)
 3 Months Ended 12 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Dec. 31, 2016
Dec. 31, 2015
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Percentage of Investments Recorded at Fair Value 93.00%   94.00%  
Fair value investments $ 5,097,600,000 $ 3,360,200,000 $ 5,039,700,000 $ 4,050,900,000
Other Investments 331,700,000   323,300,000  
Short-term investments, at amortized cost (which approximates fair value) 230,900,000   287,000,000  
Fair Value, Assets Measured on Recurring Basis Gain (Loss) Included in Earnings 54,400,000 (29,600,000)    
Accretion (Amortization) of Discounts and Premiums, Investments 6,500,000 4,200,000    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases 1,095,300,000 412,400,000    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales (1,080,100,000) (1,128,500,000)    
Transfers in 0 0    
Fair Value, Assets Measured on Recurring Basis, Transfers out 0 0    
Level 1 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 562,300,000 419,600,000 612,500,000 1,152,200,000
Fair Value, Assets Measured on Recurring Basis Gain (Loss) Included in Earnings (22,400,000) (11,300,000)    
Accretion (Amortization) of Discounts and Premiums, Investments 0 0    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases 124,500,000 109,000,000    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales (197,100,000) (852,900,000)    
Transfers in 0 0    
Fair Value, Assets Measured on Recurring Basis, Transfers out 0 0    
Level 2 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 4,155,800,000 2,566,100,000 4,118,500,000 2,531,400,000
Fair Value, Assets Measured on Recurring Basis Gain (Loss) Included in Earnings (25,500,000) (19,100,000)    
Accretion (Amortization) of Discounts and Premiums, Investments 6,500,000 4,200,000    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases 890,400,000 292,400,000    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales (866,900,000) (272,600,000)    
Transfers in 0 0    
Fair Value, Assets Measured on Recurring Basis, Transfers out 0 0    
Debt securities issued by corporations        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 1,686,800,000   1,509,400,000  
Mortgage-backed and asset-backed securities        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 2,016,400,000   2,132,900,000  
Mortgage-backed and asset-backed securities | Level 2 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 1,958,100,000   2,132,900,000  
Mortgage-backed and asset-backed securities | Level 3 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 58,300,000   0  
Fixed maturity investments. | Level 3 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 58,300,000 70,500,000 0 70,000,000
Fair Value, Assets Measured on Recurring Basis Gain (Loss) Included in Earnings (200,000) (500,000)    
Accretion (Amortization) of Discounts and Premiums, Investments 0 0    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases 58,100,000 0    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales      
Transfers in 0 0    
Fair Value, Assets Measured on Recurring Basis, Transfers out 0 0    
Common equity securities | Level 3 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 0 0 0 0
Fair Value, Assets Measured on Recurring Basis Gain (Loss) Included in Earnings 0 0    
Accretion (Amortization) of Discounts and Premiums, Investments 0 0    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases   0    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales 0      
Transfers in 0 0    
Fair Value, Assets Measured on Recurring Basis, Transfers out 0 0    
Other long-term investments | Level 3 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 174,000,000 172,700,000 177,700,000 169,500,000
Fair Value, Assets Measured on Recurring Basis Gain (Loss) Included in Earnings 1,900,000 (1,100,000)    
Accretion (Amortization) of Discounts and Premiums, Investments 0 0    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases 200,000 2,100,000    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales (2,000,000)      
Transfers in 0 0    
Fair Value, Assets Measured on Recurring Basis, Transfers out 0 0    
Hedge and private equity funds included in other long-term investments        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Other long-term investments 147,200,000   131,000,000  
Hedge and private equity funds included in other long-term investments | Level 3 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 147,200,000 131,300,000 131,000,000 127,800,000
Fair Value, Assets Measured on Recurring Basis Gain (Loss) Included in Earnings (8,200,000) 2,400,000    
Accretion (Amortization) of Discounts and Premiums, Investments 0 0    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases 22,100,000 8,900,000    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales (14,100,000) (3,000,000)    
Transfers in 0 0    
Fair Value, Assets Measured on Recurring Basis, Transfers out 0 0    
Fair value measured on a recurring basis        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 5,181,300,000   5,195,900,000  
Fair value measured on a recurring basis | Level 1 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 790,200,000   887,000,000  
Fair value measured on a recurring basis | Level 2 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 4,158,800,000   4,131,200,000  
Fair value measured on a recurring basis | Level 3 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 232,300,000   177,700,000  
Fair value measured on a recurring basis | U.S. Government and agency obligations        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 110,300,000   278,300,000  
Fair value measured on a recurring basis | U.S. Government and agency obligations | Level 1 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 100,800,000   268,800,000  
Fair value measured on a recurring basis | U.S. Government and agency obligations | Level 2 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 9,500,000   9,500,000  
Fair value measured on a recurring basis | U.S. Government and agency obligations | Level 3 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 0   0  
Fair value measured on a recurring basis | Debt securities issued by corporations        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 1,686,800,000   1,509,400,000  
Fair value measured on a recurring basis | Debt securities issued by corporations | Level 1 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 0   0  
Fair value measured on a recurring basis | Debt securities issued by corporations | Level 2 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 1,686,800,000   1,509,400,000  
Fair value measured on a recurring basis | Debt securities issued by corporations | Level 3 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 0   0  
Fair value measured on a recurring basis | Debt securities issued by corporations | Consumer        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 362,900,000   385,600,000  
Fair value measured on a recurring basis | Debt securities issued by corporations | Consumer | Level 1 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 0   0  
Fair value measured on a recurring basis | Debt securities issued by corporations | Consumer | Level 2 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 362,900,000   385,600,000  
Fair value measured on a recurring basis | Debt securities issued by corporations | Consumer | Level 3 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 0   0  
Fair value measured on a recurring basis | Debt securities issued by corporations | Industrial        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 174,500,000   146,400,000  
Fair value measured on a recurring basis | Debt securities issued by corporations | Industrial | Level 1 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 0   0  
Fair value measured on a recurring basis | Debt securities issued by corporations | Industrial | Level 2 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 174,500,000   146,400,000  
Fair value measured on a recurring basis | Debt securities issued by corporations | Industrial | Level 3 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 0   0  
Fair value measured on a recurring basis | Debt securities issued by corporations | Financials        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 263,300,000   176,000,000  
Fair value measured on a recurring basis | Debt securities issued by corporations | Financials | Level 1 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 0   0  
Fair value measured on a recurring basis | Debt securities issued by corporations | Financials | Level 2 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 263,300,000   176,000,000  
Fair value measured on a recurring basis | Debt securities issued by corporations | Financials | Level 3 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 0   0  
Fair value measured on a recurring basis | Debt securities issued by corporations | Communications [Member]        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 135,900,000   131,400,000  
Fair value measured on a recurring basis | Debt securities issued by corporations | Communications [Member] | Level 1 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 0   0  
Fair value measured on a recurring basis | Debt securities issued by corporations | Communications [Member] | Level 2 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 135,900,000   131,400,000  
Fair value measured on a recurring basis | Debt securities issued by corporations | Communications [Member] | Level 3 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 0   0  
Fair value measured on a recurring basis | Debt securities issued by corporations | Health Care [Member]        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 248,400,000   244,200,000  
Fair value measured on a recurring basis | Debt securities issued by corporations | Health Care [Member] | Level 1 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 0   0  
Fair value measured on a recurring basis | Debt securities issued by corporations | Health Care [Member] | Level 2 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 248,400,000   244,200,000  
Fair value measured on a recurring basis | Debt securities issued by corporations | Health Care [Member] | Level 3 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 0   0  
Fair value measured on a recurring basis | Debt securities issued by corporations | Energy        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 52,500,000   53,500,000  
Fair value measured on a recurring basis | Debt securities issued by corporations | Energy | Level 1 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 0   0  
Fair value measured on a recurring basis | Debt securities issued by corporations | Energy | Level 2 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 52,500,000   53,500,000  
Fair value measured on a recurring basis | Debt securities issued by corporations | Energy | Level 3 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 0   0  
Fair value measured on a recurring basis | Debt securities issued by corporations | Technology        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 118,100,000   89,400,000  
Fair value measured on a recurring basis | Debt securities issued by corporations | Technology | Level 1 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 0   0  
Fair value measured on a recurring basis | Debt securities issued by corporations | Technology | Level 2 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 118,100,000   89,400,000  
Fair value measured on a recurring basis | Debt securities issued by corporations | Technology | Level 3 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 0   0  
Fair value measured on a recurring basis | Debt securities issued by corporations | Utilities        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 229,300,000   180,300,000  
Fair value measured on a recurring basis | Debt securities issued by corporations | Utilities | Level 1 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 0   0  
Fair value measured on a recurring basis | Debt securities issued by corporations | Utilities | Level 2 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 229,300,000   180,300,000  
Fair value measured on a recurring basis | Debt securities issued by corporations | Utilities | Level 3 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 0   0  
Fair value measured on a recurring basis | Debt securities issued by corporations | Materials        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 101,900,000   102,600,000  
Fair value measured on a recurring basis | Debt securities issued by corporations | Materials | Level 1 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 0   0  
Fair value measured on a recurring basis | Debt securities issued by corporations | Materials | Level 2 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 101,900,000   102,600,000  
Fair value measured on a recurring basis | Debt securities issued by corporations | Materials | Level 3 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 0   0  
Fair value measured on a recurring basis | Mortgage-backed and asset-backed securities        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 2,016,400,000   2,132,900,000  
Fair value measured on a recurring basis | Mortgage-backed and asset-backed securities | Level 1 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 0   0  
Fair value measured on a recurring basis | Mortgage-backed and asset-backed securities | Level 2 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 1,958,100,000   2,132,900,000  
Fair value measured on a recurring basis | Mortgage-backed and asset-backed securities | Level 3 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 58,300,000   0  
Fair value measured on a recurring basis | Municipal obligations        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 330,300,000   309,000,000  
Fair value measured on a recurring basis | Municipal obligations | Level 1 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 0   0  
Fair value measured on a recurring basis | Municipal obligations | Level 2 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 330,300,000   309,000,000  
Fair value measured on a recurring basis | Municipal obligations | Level 3 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 0   0  
Fair value measured on a recurring basis | Foreign government, agency and provincial obligations        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 17,900,000   13,200,000  
Fair value measured on a recurring basis | Foreign government, agency and provincial obligations | Level 1 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 600,000   600,000  
Fair value measured on a recurring basis | Foreign government, agency and provincial obligations | Level 2 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 17,300,000   12,600,000  
Fair value measured on a recurring basis | Foreign government, agency and provincial obligations | Level 3 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 0   0  
Fair value measured on a recurring basis | Preferred Stock        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 13,700,000   14,000,000  
Fair value measured on a recurring basis | Preferred Stock | Level 1 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 0   0  
Fair value measured on a recurring basis | Preferred Stock | Level 2 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 13,700,000   14,000,000  
Fair value measured on a recurring basis | Preferred Stock | Level 3 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 0   0  
Fair value measured on a recurring basis | Fixed maturity investments.        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 4,175,400,000   4,256,800,000  
Fair value measured on a recurring basis | Fixed maturity investments. | Level 1 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 101,400,000   269,400,000  
Fair value measured on a recurring basis | Fixed maturity investments. | Level 2 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 4,015,700,000   3,987,400,000  
Fair value measured on a recurring basis | Fixed maturity investments. | Level 3 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 58,300,000   0  
Fair value measured on a recurring basis | Short-term investments        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 230,900,000   287,100,000  
Fair value measured on a recurring basis | Short-term investments | Level 1 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 227,900,000   274,400,000  
Short-term investments, at amortized cost (which approximates fair value)   211,300,000 287,100,000 261,700,000
Fair value measured on a recurring basis | Short-term investments | Level 2 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 3,000,000   12,700,000  
Fair value measured on a recurring basis | Short-term investments | Level 3 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 0   0  
Fair value measured on a recurring basis | Common equity securities | Consumer        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 32,200,000   12,900,000  
Fair value measured on a recurring basis | Common equity securities | Consumer | Level 1 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 32,200,000   12,900,000  
Fair value measured on a recurring basis | Common equity securities | Consumer | Level 2 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 0   0  
Fair value measured on a recurring basis | Common equity securities | Consumer | Level 3 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 0   0  
Fair value measured on a recurring basis | Common equity securities | Industrial        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 9,000,000   2,200,000  
Fair value measured on a recurring basis | Common equity securities | Industrial | Level 1 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 9,000,000   2,200,000  
Fair value measured on a recurring basis | Common equity securities | Industrial | Level 2 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 0   0  
Fair value measured on a recurring basis | Common equity securities | Industrial | Level 3 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 0   0  
Fair value measured on a recurring basis | Common equity securities | Financials        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 18,200,000   11,600,000  
Fair value measured on a recurring basis | Common equity securities | Financials | Level 1 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 18,200,000   11,600,000  
Fair value measured on a recurring basis | Common equity securities | Financials | Level 2 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 0   0  
Fair value measured on a recurring basis | Common equity securities | Financials | Level 3 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 0   0  
Fair value measured on a recurring basis | Common equity securities | Communications [Member]        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 13,900,000   10,500,000  
Fair value measured on a recurring basis | Common equity securities | Communications [Member] | Level 1 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 13,900,000   10,500,000  
Fair value measured on a recurring basis | Common equity securities | Communications [Member] | Level 2 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 0   0  
Fair value measured on a recurring basis | Common equity securities | Communications [Member] | Level 3 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 0   0  
Fair value measured on a recurring basis | Common equity securities | Health Care [Member]        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 27,100,000   20,900,000  
Fair value measured on a recurring basis | Common equity securities | Health Care [Member] | Level 1 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 27,100,000   20,900,000  
Fair value measured on a recurring basis | Common equity securities | Health Care [Member] | Level 2 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 0   0  
Fair value measured on a recurring basis | Common equity securities | Health Care [Member] | Level 3 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 0   0  
Fair value measured on a recurring basis | Common equity securities | Energy        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 7,700,000   3,700,000  
Fair value measured on a recurring basis | Common equity securities | Energy | Level 1 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 7,700,000   3,700,000  
Fair value measured on a recurring basis | Common equity securities | Energy | Level 2 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 0   0  
Fair value measured on a recurring basis | Common equity securities | Energy | Level 3 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 0   0  
Fair value measured on a recurring basis | Common equity securities | Technology        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 16,100,000   11,000,000  
Fair value measured on a recurring basis | Common equity securities | Technology | Level 1 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 16,100,000   11,000,000  
Fair value measured on a recurring basis | Common equity securities | Technology | Level 2 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 0   0  
Fair value measured on a recurring basis | Common equity securities | Technology | Level 3 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 0   0  
Fair value measured on a recurring basis | Common equity securities | Utilities        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 1,000,000      
Fair value measured on a recurring basis | Common equity securities | Utilities | Level 1 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 1,000,000      
Fair value measured on a recurring basis | Common equity securities | Utilities | Level 2 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 0      
Fair value measured on a recurring basis | Common equity securities | Utilities | Level 3 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 0      
Fair value measured on a recurring basis | Common equity securities | Materials        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 5,100,000      
Fair value measured on a recurring basis | Common equity securities | Materials | Level 1 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 5,100,000      
Fair value measured on a recurring basis | Common equity securities | Materials | Level 2 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 0      
Fair value measured on a recurring basis | Common equity securities | Materials | Level 3 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 0      
Fair value measured on a recurring basis | Common equity securities | Exchange Traded Funds [Member]        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 384,800,000   321,600,000  
Fair value measured on a recurring basis | Common equity securities | Exchange Traded Funds [Member] | Level 1 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 330,600,000   270,400,000  
Fair value measured on a recurring basis | Common equity securities | Exchange Traded Funds [Member] | Level 2 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 54,200,000   51,200,000  
Fair value measured on a recurring basis | Common equity securities | Exchange Traded Funds [Member] | Level 3 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 0   0  
Fair value measured on a recurring basis | Common equity securities | Other:        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 85,900,000   79,900,000  
Fair value measured on a recurring basis | Common equity securities | Other: | Level 1 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 0   0  
Fair value measured on a recurring basis | Common equity securities | Other: | Level 2 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 85,900,000   79,900,000  
Fair value measured on a recurring basis | Common equity securities | Other: | Level 3 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 0   0  
Fair value measured on a recurring basis | Common equity securities | Common equity securities        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 601,000,000   474,300,000  
Fair value measured on a recurring basis | Common equity securities | Common equity securities | Level 1 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 460,900,000   343,200,000  
Fair value measured on a recurring basis | Common equity securities | Common equity securities | Level 2 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 140,100,000   131,100,000  
Fair value measured on a recurring basis | Common equity securities | Common equity securities | Level 3 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 0   0  
Fair value measured on a recurring basis | Other long-term investments        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 174,000,000   177,700,000  
Fair value measured on a recurring basis | Other long-term investments | Level 1 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 0   0  
Fair value measured on a recurring basis | Other long-term investments | Level 2 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 0   0  
Fair value measured on a recurring basis | Other long-term investments | Level 3 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 174,000,000   177,700,000  
Carrying value of investment accounted for using the equity method 3,000,000 3,700,000 3,500,000 3,800,000
Affordable Housing Development Fund        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Other Investments 11,700,000 14,100,000 12,300,000 14,700,000
Affordable Housing Development Fund | Fair value measured on a recurring basis | Other long-term investments        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Other Investments 11,700,000   12,300,000  
Forward Contracts | Fair value measured on a recurring basis | Other long-term investments | Level 3 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Derivative, fair value, net $ (4,200,000.0)   (1,200,000.0)  
Star & Shield Insurance Exchange        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Trading Securities Debt Reclassified to Assets Held-for-sale   $ 9,400,000 6,600,000 $ 9,500,000
Star & Shield Insurance Exchange | Short-term investments        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Trading Securities Debt Reclassified to Assets Held-for-sale     $ 100,000  
Surplus Note        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
RBC Score 250.00%   250.00%  
Star & Shield Insurance Exchange        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales $ (5,200,000)      
Star & Shield Insurance Exchange | Level 1 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales 0      
Star & Shield Insurance Exchange | Level 2 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales (5,200,000)      
Star & Shield Insurance Exchange | Fixed maturity investments. | Level 3 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales 0      
Star & Shield Insurance Exchange | Common equity securities | Level 3 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales 0      
Star & Shield Insurance Exchange | Other long-term investments | Level 3 Inputs        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales $ 0