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Summary of Significant Accounting Policies (Details) - Additional Details
$ in Millions
12 Months Ended
Dec. 31, 2019
USD ($)
businesses
segment
Dec. 31, 2018
USD ($)
Dec. 31, 2017
Accounting Policies [Abstract]      
Minimum collateral provided by borrowers of securities, as a percentage of the market value of the loaned securities plus accrued interest 102.00%    
Number of reporting units for which the Company performs a review of goodwill held | segment 3    
Number of operating segments | segment 3    
Number of reportable segments | segment 3    
Liability for guaranty fund and other insurance-related assessments | $ $ 184 $ 217  
Recoverables for liability for guaranty fund and other insurance-related assessments | $ $ 13 $ 16  
Minimum expected payment period for loss-based assessments and recoveries 1 year 1 year  
Net written premiums for participating dividend policies as a percent of total Company net written premiums 1.00% 1.00% 1.00%
Liability accrued for policyholder dividends | $ $ 71 $ 72  
Percentage of capital provided by International for its syndicate at Lloyd's 100.00%    
Number of principal business units through which the Company's syndicate at Lloyd's writes business | businesses 6