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Fair Value Measurements - Fair Value Inputs (Details) - USD ($)
 3 Months Ended
Mar. 31, 2023
Mar. 31, 2022
Dec. 31, 2022
Dec. 31, 2021
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Fixed maturities at fair value $ 7,189,400,000   $ 6,894,800,000  
Equity securities 243,600,000   243,200,000  
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases 19,400,000 $ 13,700,000    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3 400,000 5,300,000    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value 210,200,000 218,900,000 230,000,000.0 $ 251,400,000
Included in Condensed Consolidated Statement of Operations (100,000) (5,700,000)    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3 0 0    
Included in Other Comprehensive Income (Loss) 1,700,000 (3,000,000.0)    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements 0 0    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales (41,200,000) (42,800,000)    
Equity Securities        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases 0 0    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3 400,000 0    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value 2,000,000.0 1,500,000 2,100,000 1,500,000
Included in Condensed Consolidated Statement of Operations (500,000) 0    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3 0 0    
Included in Other Comprehensive Income (Loss) 0 0    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements 0 0    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales 0 0    
Redeemable Preferred Stocks        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases 0 0    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3   0    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value 6,800,000 5,600,000 6,800,000 $ 6,100,000
Included in Condensed Consolidated Statement of Operations 0 0    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3   0    
Included in Other Comprehensive Income (Loss) 0 (500,000)    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements 0 0    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales 0 $ 0    
Fair Value, Measurements, Recurring        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Convertible debt at fair value 45,300,000   43,300,000  
Total Assets 7,486,300,000   7,183,000,000  
Derivative Asset 4,500,000   1,700,000  
Liabilities, Fair Value Disclosure     (400,000)  
Fair Value, Measurements, Recurring | Investments in Fixed Maturities        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Fixed maturities at fair value 7,189,400,000   6,894,800,000  
Fair Value, Measurements, Recurring | U.S. Government and Government Agencies and Authorities        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Fixed maturities at fair value 564,000,000.0   528,000,000.0  
Fair Value, Measurements, Recurring | States and Political Subdivisions        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Fixed maturities at fair value 1,622,900,000   1,568,900,000  
Fair Value, Measurements, Recurring | Foreign Governments        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Fixed maturities at fair value 4,000,000.0   4,100,000  
Fair Value, Measurements, Recurring | Bonds and Notes        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Fixed maturities at fair value 3,703,300,000   3,539,400,000  
Fair Value, Measurements, Recurring | Redeemable Preferred Stocks        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Fixed maturities at fair value 8,000,000.0      
Fair Value, Measurements, Recurring | Redeemable Preferred Stocks        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Fixed maturities at fair value     8,000,000.0  
Fair Value, Measurements, Recurring | Collateralized Loan Obligations        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Fixed maturities at fair value 981,900,000   953,900,000  
Fair Value, Measurements, Recurring | Other Mortgage- and Asset-backed        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Fixed maturities at fair value 305,300,000   292,500,000  
Fair Value, Measurements, Recurring | Equity Securities        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Equity securities 243,600,000   243,200,000  
Fair Value, Measurements, Recurring | Exchange Traded Funds        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Equity securities 6,800,000   12,200,000  
Fair Value, Measurements, Recurring | Limited Liability Companies and Limited Partnerships        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Equity securities 195,600,000   189,100,000  
Fair Value, Measurements, Recurring | Designated as Hedging Instrument        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Derivative Liability     (400,000)  
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Convertible debt at fair value 0      
Total Assets 128,400,000   117,000,000.0  
Convertible Securities, Fair Value Disclosures     0  
Derivative Asset 0   0  
Liabilities, Fair Value Disclosure     0  
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Investments in Fixed Maturities        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Fixed maturities at fair value 120,800,000   103,600,000  
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. Government and Government Agencies and Authorities        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Fixed maturities at fair value 120,800,000   103,600,000  
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | States and Political Subdivisions        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Fixed maturities at fair value 0   0  
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Foreign Governments        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Fixed maturities at fair value 0   0  
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Bonds and Notes        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Fixed maturities at fair value 0   0  
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Redeemable Preferred Stocks        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Fixed maturities at fair value 0      
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Redeemable Preferred Stocks        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Fixed maturities at fair value     0  
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Collateralized Loan Obligations        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Fixed maturities at fair value 0   0  
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other Mortgage- and Asset-backed        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Fixed maturities at fair value 0   0  
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Equity Securities        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Equity securities 7,600,000   13,400,000  
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Exchange Traded Funds        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Equity securities 6,800,000   12,200,000  
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Limited Liability Companies and Limited Partnerships        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Equity securities 0   0  
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Designated as Hedging Instrument        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Derivative Liability     0  
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2)        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Convertible debt at fair value 45,300,000   43,300,000  
Total Assets 6,952,100,000   6,646,900,000  
Derivative Asset     1,700,000  
Derivative Liability     (400,000)  
Liabilities, Fair Value Disclosure     (400,000)  
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Investments in Fixed Maturities        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Fixed maturities at fair value 6,860,400,000   6,563,300,000  
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | U.S. Government and Government Agencies and Authorities        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Fixed maturities at fair value 443,200,000   424,400,000  
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | States and Political Subdivisions        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Fixed maturities at fair value 1,622,900,000   1,568,900,000  
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Foreign Governments        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Fixed maturities at fair value 4,000,000.0   4,100,000  
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Bonds and Notes        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Fixed maturities at fair value 3,507,200,000   3,323,400,000  
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Redeemable Preferred Stocks        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Fixed maturities at fair value 1,200,000      
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Redeemable Preferred Stocks        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Fixed maturities at fair value     1,200,000  
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Collateralized Loan Obligations        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Fixed maturities at fair value 981,900,000   953,900,000  
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Other Mortgage- and Asset-backed        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Fixed maturities at fair value 300,000,000.0   287,400,000  
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Equity Securities        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Equity securities 38,400,000   38,600,000  
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Exchange Traded Funds        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Equity securities 0   0  
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Limited Liability Companies and Limited Partnerships        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Equity securities 0   0  
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3)        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Convertible debt at fair value 0   0  
Total Assets 210,200,000   230,000,000.0  
Derivative Asset 0   0  
Liabilities, Fair Value Disclosure     0  
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Investments in Fixed Maturities        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Fixed maturities at fair value 208,200,000   227,900,000  
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | U.S. Government and Government Agencies and Authorities        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Fixed maturities at fair value 0   0  
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | States and Political Subdivisions        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Fixed maturities at fair value 0   0  
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Foreign Governments        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Fixed maturities at fair value 0   0  
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Bonds and Notes        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Fixed maturities at fair value 196,100,000   216,000,000.0  
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Redeemable Preferred Stocks        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Fixed maturities at fair value 6,800,000      
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Redeemable Preferred Stocks        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Fixed maturities at fair value     6,800,000  
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Collateralized Loan Obligations        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Fixed maturities at fair value 0   0  
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Other Mortgage- and Asset-backed        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Fixed maturities at fair value 5,300,000   5,100,000  
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Equity Securities        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Equity securities 2,000,000.0   2,100,000  
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Exchange Traded Funds        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Equity securities 0   0  
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Limited Liability Companies and Limited Partnerships        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Equity securities 0   0  
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Designated as Hedging Instrument        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Derivative Liability     0  
Finance, Insurance and Real Estate | Fair Value, Measurements, Recurring | Preferred Stocks        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Equity securities 28,800,000   29,000,000.0  
Finance, Insurance and Real Estate | Fair Value, Measurements, Recurring | Common Stock        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Equity securities 600,000   900,000  
Finance, Insurance and Real Estate | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Preferred Stocks        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Equity securities 0   0  
Finance, Insurance and Real Estate | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Common Stock        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Equity securities 600,000   900,000  
Finance, Insurance and Real Estate | Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Preferred Stocks        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Equity securities 28,800,000   29,000,000.0  
Finance, Insurance and Real Estate | Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Common Stock        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Equity securities 0   0  
Finance, Insurance and Real Estate | Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Preferred Stocks        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Equity securities 0   0  
Finance, Insurance and Real Estate | Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Common Stock        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Equity securities 0   0  
Other Industries | Fair Value, Measurements, Recurring | Preferred Stocks        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Equity securities 11,200,000   10,800,000  
Other Industries | Fair Value, Measurements, Recurring | Common Stock        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Equity securities 600,000   1,200,000  
Other Industries | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Preferred Stocks        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Equity securities 0   0  
Other Industries | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Common Stock        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Equity securities 200,000   300,000  
Other Industries | Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Preferred Stocks        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Equity securities 9,600,000   9,200,000  
Other Industries | Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Common Stock        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Equity securities 0   400,000  
Other Industries | Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Preferred Stocks        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Equity securities 1,600,000   1,600,000  
Other Industries | Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Common Stock        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Equity securities 400,000   500,000  
Measured at Net Asset Value | Fair Value, Measurements, Recurring        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Convertible debt at fair value 0   0  
Total Assets 195,600,000   189,100,000  
Derivative Asset 0   0  
Liabilities, Fair Value Disclosure     0  
Measured at Net Asset Value | Fair Value, Measurements, Recurring | Investments in Fixed Maturities        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Fixed maturities at fair value 0   0  
Measured at Net Asset Value | Fair Value, Measurements, Recurring | U.S. Government and Government Agencies and Authorities        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Fixed maturities at fair value 0   0  
Measured at Net Asset Value | Fair Value, Measurements, Recurring | States and Political Subdivisions        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Fixed maturities at fair value 0   0  
Measured at Net Asset Value | Fair Value, Measurements, Recurring | Foreign Governments        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Fixed maturities at fair value 0   0  
Measured at Net Asset Value | Fair Value, Measurements, Recurring | Bonds and Notes        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Fixed maturities at fair value 0   0  
Measured at Net Asset Value | Fair Value, Measurements, Recurring | Redeemable Preferred Stocks        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Fixed maturities at fair value 0      
Measured at Net Asset Value | Fair Value, Measurements, Recurring | Redeemable Preferred Stocks        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Fixed maturities at fair value     0  
Measured at Net Asset Value | Fair Value, Measurements, Recurring | Collateralized Loan Obligations        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Fixed maturities at fair value 0   0  
Measured at Net Asset Value | Fair Value, Measurements, Recurring | Other Mortgage- and Asset-backed        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Fixed maturities at fair value 0   0  
Measured at Net Asset Value | Fair Value, Measurements, Recurring | Equity Securities        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Equity securities 195,600,000   189,100,000  
Measured at Net Asset Value | Fair Value, Measurements, Recurring | Exchange Traded Funds        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Equity securities 0   0  
Measured at Net Asset Value | Fair Value, Measurements, Recurring | Limited Liability Companies and Limited Partnerships        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Equity securities 195,600,000   189,100,000  
Measured at Net Asset Value | Fair Value, Measurements, Recurring | Designated as Hedging Instrument        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Derivative Liability     0  
Measured at Net Asset Value | Finance, Insurance and Real Estate | Fair Value, Measurements, Recurring | Preferred Stocks        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Equity securities 0   0  
Measured at Net Asset Value | Finance, Insurance and Real Estate | Fair Value, Measurements, Recurring | Common Stock        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Equity securities 0   0  
Measured at Net Asset Value | Other Industries | Fair Value, Measurements, Recurring | Preferred Stocks        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Equity securities 0   0  
Measured at Net Asset Value | Other Industries | Fair Value, Measurements, Recurring | Common Stock        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Equity securities $ 0   $ 0