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Basis of Presentation and Significant Accounting Policies Additional Information (Details) - USD ($)
$ / shares in Units, $ in Millions
 12 Months Ended
Apr. 29, 1991
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Dec. 31, 2017
Dec. 31, 2020
Apr. 30, 2013
Finite-Lived Intangible Assets [Line Items]              
Operating Lease, Liability   $ 22 $ 25        
Income Taxes Paid   $ 11 $ 6 $ 15      
Common Stock Voting Restrictions 10.00%            
Common Stock, Par or Stated Value Per Share             $ 0.01
Preferred stock, shares authorized   20,000,000 20,000,000        
Preferred Stock, Par or Stated Value Per Share   $ 0.01         $ 0.01
Common stock, shares issued   46,659,144 46,658,990        
Warrant Exercised During Period Shares   1,503 0 132      
Common Stock Transfer Restrictions 5.00%            
Common Stock Additional Transfer Restrictions         5.00%    
Restricted Cash   $ 12 $ 14 $ 5      
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents   274 61 23   $ 35  
Investment income due and accrued   $ 14 $ 10        
Accrued Investment Interest Receivable Write-off Period in Days   90          
Weighted average period of future premiums   7 years 8 months 12 days 8 years        
Estimated Future Premium Payments Weighted Average Discounted Rate   3.20% 3.00%        
Investment income due and accrued   $ 14 $ 10        
CECL Securities Factor for Consideration   20.00%          
Specialty Property & Casualty Program              
Finite-Lived Intangible Assets [Line Items]              
Reinsurance, Reinsured Risk, Percentage   70.00%          
Xchange              
Finite-Lived Intangible Assets [Line Items]              
Business Acquisition, Percentage of Voting Interests Acquired   80.00%          
All Trans              
Finite-Lived Intangible Assets [Line Items]              
Business Acquisition, Percentage of Voting Interests Acquired   85.00%          
Capacity Marine              
Finite-Lived Intangible Assets [Line Items]              
Business Acquisition, Percentage of Voting Interests Acquired   80.00%          
Riverton              
Finite-Lived Intangible Assets [Line Items]              
Business Acquisition, Percentage of Voting Interests Acquired   80.00%          
Minimum [Member] | Furniture and Fixtures [Member]              
Finite-Lived Intangible Assets [Line Items]              
Finite-Lived Intangible Asset, Useful Life   3 years          
Maximum [Member] | Furniture and Fixtures [Member]              
Finite-Lived Intangible Assets [Line Items]              
Finite-Lived Intangible Asset, Useful Life   5 years          
Long-term Debt [Member]              
Finite-Lived Intangible Assets [Line Items]              
Interest Paid, Including Capitalized Interest, Operating and Investing Activities   $ 50 283 80      
Variable Interest Entity, Primary Beneficiary [Member]              
Finite-Lived Intangible Assets [Line Items]              
Restricted Cash   $ 246 $ 17 $ 2      
Estimated Future Premium Payments Weighted Average Discounted Rate   6.30% 6.80%