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Business Acquisitions - Additional Information (Details) - USD ($)
$ / shares in Units, $ in Millions
 3 Months Ended 7 Months Ended 12 Months Ended 19 Months Ended
Dec. 31, 2020
May 23, 2019
Dec. 31, 2020
Sep. 30, 2020
Jun. 30, 2020
Mar. 31, 2020
Dec. 31, 2019
Sep. 30, 2019
Jun. 30, 2019
Mar. 31, 2019
Dec. 31, 2019
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2020
Business Acquisition [Line Items]                              
Goodwill, Purchase Accounting Adjustments                       $ (2)      
Goodwill $ 1,911   $ 1,911       $ 1,913       $ 1,913 1,911 $ 1,913 $ 1,290 $ 1,911
Revenues     5,328 $ 5,171 $ 5,068 $ 4,956 5,361 $ 5,347 $ 5,092 $ 4,940   20,523 20,740 18,955  
Net income (loss) available to common stockholders     532 $ 453 463 $ 268 543 $ 524 372 $ 625   1,716 2,064 1,801  
Net investment income                       1,846 1,951 $ 1,780  
Adverse Development Cover Navigators Group [Member]                              
Business Acquisition [Line Items]                              
Reinsurance premium             $ 91                
Adverse development from comprehensive annual review                       209      
Change in Deferred Gain on Retroactive Reinsurance                       102 16    
Adverse Development Cover Navigators Group [Member] | Remaining [Member]                              
Business Acquisition [Line Items]                              
Reinsurance Retention Policy, Excess Retention, Amount Reinsured 91                            
Adverse Development Cover Navigators Group [Member] | Maximum                              
Business Acquisition [Line Items]                              
Reinsurance Retention Policy, Excess Retention, Amount Reinsured                       300      
Adverse Development Cover Navigators Group [Member] | Retention Layer Above Reserve for the Covered Liabilities as of the Inception Date [Member]                              
Business Acquisition [Line Items]                              
Reinsurance Retention Policy, Amount Retained                       100      
Adverse development from comprehensive annual review                       309      
Adverse Development Cover Navigators Group [Member] | Retention Layer for Reserve for the Covered Liabilities as of the Inception Date [Member]                              
Business Acquisition [Line Items]                              
Reinsurance Retention Policy, Amount Retained                       1,816      
The Navigators Group, Inc. [Member]                              
Business Acquisition [Line Items]                              
Business Acquisition, Effective Date of Acquisition   May 23, 2019                          
Business Acquisition, Name of Acquired Entity   Navigators Group                          
Business Acquisition, Share Price   $ 70                          
Payments to Acquire Businesses, Gross   $ 2,121                          
Business Combination, Consideration Transferred, Other   23                          
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Total Assets         9                    
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Total Liabilities         7                    
Goodwill, Purchase Accounting Adjustments         $ (2)                    
Goodwill 621 623 $ 621                 621     621
Revenues                     1,000        
Net income (loss) available to common stockholders                     167        
Net investment income                     67        
Net Investment Income, after Tax                     54        
Business Combination, Acquisition Related Costs                         17    
The Navigators Group, Inc. [Member] | Adverse Development Cover Navigators Group [Member]                              
Business Acquisition [Line Items]                              
Reinsurance premium                     91   91    
Reinsurance Retention Policy, Amount Retained                 68       68    
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims                 97   97   84    
Current accident year                 $ 29            
Use of Retention Layer by Acquiree Prior to Acquisition   32                          
Adverse development from comprehensive annual review                       209 91    
Change in Deferred Gain on Retroactive Reinsurance                     16 102     $ 118
The Navigators Group, Inc. [Member] | Adverse Development Cover Navigators Group [Member] | After tax [Member]                              
Business Acquisition [Line Items]                              
Reinsurance premium                     72        
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims                     77        
Change in Deferred Gain on Retroactive Reinsurance                     $ 13 81      
The Navigators Group, Inc. [Member] | Adverse Development Cover Navigators Group [Member] | Remaining [Member]                              
Business Acquisition [Line Items]                              
Reinsurance Retention Policy, Excess Retention, Amount Reinsured $ 91                            
The Navigators Group, Inc. [Member] | Adverse Development Cover Navigators Group [Member] | Maximum                              
Business Acquisition [Line Items]                              
Reinsurance Retention Policy, Excess Retention, Amount Reinsured                       $ 300 300    
The Navigators Group, Inc. [Member] | Adverse Development Cover Navigators Group [Member] | Retention Layer Above Reserve for the Covered Liabilities as of the Inception Date [Member]                              
Business Acquisition [Line Items]                              
Reinsurance Retention Policy, Amount Retained                         100    
The Navigators Group, Inc. [Member] | Adverse Development Cover Navigators Group [Member] | Retention Layer for Reserve for the Covered Liabilities as of the Inception Date [Member]                              
Business Acquisition [Line Items]                              
Reinsurance Retention Policy, Amount Retained                         $ 1,816    
The Navigators Group, Inc. [Member] | Recorded by Acquiree Prior to Acquisition [Member]                              
Business Acquisition [Line Items]                              
Current accident year   52                          
The Navigators Group, Inc. [Member] | Cash                              
Business Acquisition [Line Items]                              
Payments to Acquire Businesses, Gross   $ 2,098