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Summary Of Significant Accounting Policies (Narrative) (Details)
 3 Months Ended 12 Months Ended
Mar. 31, 2025
USD ($)
Dec. 31, 2024
USD ($)
state
subsidiary
independent_insurance_agency
reporting_locations
Dec. 31, 2023
USD ($)
Dec. 31, 2022
USD ($)
Aug. 31, 2008
Significant Accounting Policies          
Income Tax Examination, Penalties Expense   $ 0      
Number of operating subsidaries | subsidiary   2      
Number of independent insurance agencies | independent_insurance_agency   2,175      
Number of reporting locations | reporting_locations   3,355      
Number of states in which entity operates | state   46      
Excess (deficit) of Separate Account Fair Value over (under) Contract Holder Account Value   $ (42,000,000) $ (43,000,000)    
Other invested assets   713,000,000 577,000,000    
Depreciation expense   34,000,000 30,000,000 $ 33,000,000  
Land, building and equipment impairments   0 0 0  
Premium Receivable, Allowance for Credit Loss   18,000,000 16,000,000 13,000,000  
Minimum participant age to elect         40 years
Gross losses   9,937,000,000      
Earned premiums   $ 8,889,000,000 7,958,000,000 $ 7,225,000,000  
Property Casualty Insurance Segment          
Significant Accounting Policies          
Number of operating subsidaries | subsidiary   2      
Minimum          
Significant Accounting Policies          
Estimated useful lives   3 years      
Minimum | CA Wildfire | Subsequent Event [Member]          
Significant Accounting Policies          
Catastrophe losses net of reinsurance recoveries $ 450,000,000        
Gross losses 950,000,000        
Net reduction in earned premium 50,000,000        
Maximum          
Significant Accounting Policies          
Estimated useful lives   39 years 6 months      
Maximum | CA Wildfire | Subsequent Event [Member]          
Significant Accounting Policies          
Catastrophe losses net of reinsurance recoveries 525,000,000        
Gross losses 1,150,000,000        
Net reduction in earned premium $ 60,000,000        
Life Policy Loans          
Significant Accounting Policies          
Other invested assets   $ 36,000,000 33,000,000    
Venture Capital Funds          
Significant Accounting Policies          
Other invested assets   567,000,000 434,000,000    
Real Estate Investment          
Significant Accounting Policies          
Other invested assets   94,000,000 66,000,000    
Other than Securities Investment          
Significant Accounting Policies          
Other invested assets   $ 16,000,000 $ 44,000,000    
10 Largest States          
Significant Accounting Policies          
Percentage Of Premiums   50.10% 50.10%    
Ohio, Largest State          
Significant Accounting Policies          
Percentage Of Premiums   13.10% 13.40%    
Illinois, New York, North Carolina, Pennsylvania | Minimum          
Significant Accounting Policies          
Percentage Of Premiums   4.00%      
Illinois, New York, North Carolina, Pennsylvania | Maximum          
Significant Accounting Policies          
Percentage Of Premiums   6.00%      
Largest Single Agency Relationship          
Significant Accounting Policies          
Percentage Of Premiums   0.50%      
All Other | Maximum          
Significant Accounting Policies          
Percentage Of Premiums   8.00%      
Stock Options          
Significant Accounting Policies          
Weighted-average remaining contractual life for options expected to vest   10 years      
Personal Insurance | CA Wildfire | Subsequent Event [Member]          
Significant Accounting Policies          
Catastrophe Loss Percentage of Losses Attributable to Catastrophe Event 73.00%        
Cincinnati Reinsurance | CA Wildfire | Subsequent Event [Member]          
Significant Accounting Policies          
Catastrophe Loss Percentage of Losses Attributable to Catastrophe Event 24.00%        
Reinsurance, Excess Retention, Amount Reinsured, Per Event $ 38,000,000        
Reinsurance, Amount Retained, Per Event $ 80,000,000        
Cincinnati Global | CA Wildfire | Subsequent Event [Member]          
Significant Accounting Policies          
Catastrophe Loss Percentage of Losses Attributable to Catastrophe Event 3.00%        
Stock Options          
Significant Accounting Policies          
Awards vesting period   3 years      
Restricted Stock Units (RSUs)          
Significant Accounting Policies          
Awards vesting period   3 years      
Cincinnati Global          
Significant Accounting Policies          
Goodwill   $ 30,000,000 $ 30,000,000    
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Indefinite-Lived Intangible Assets   $ 31,000,000 $ 31,000,000    
Broker Relationships and Internally developed technology | Cincinnati Global | Minimum          
Significant Accounting Policies          
Useful life   5 years      
Broker Relationships and Internally developed technology | Cincinnati Global | Maximum          
Significant Accounting Policies          
Useful life   15 years