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Fair Value Measurement - Narrative (Details)
$ in Millions
9 Months Ended 12 Months Ended
Sep. 30, 2023
USD ($)
Security
Trust
Dec. 31, 2022
USD ($)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Repurchase agreement liability   $ 35
Assured IM    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Obligation to purchase debt   0.05
Total    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Percentage of CDS contracts fair valued Using minimum premium 11.20%  
AGC And AGM    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Number of custodial trusts | Trust 4  
Share value, amount $ 200  
Maximum amount $ 50  
Recurring | Level 3    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Number of fixed maturity securities valued using model processes | Security 189  
Recurring | Level 3 | CDR | Total | Minimum    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Conditional prepayment rate 3.62% 2.78%
Recurring | Level 3 | CDR | Total | Maximum    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Conditional prepayment rate 5.41% 5.08%