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Business Acquisitions - Additional Information (Details) - USD ($)
$ / shares in Units, $ in Millions
 1 Months Ended 3 Months Ended 6 Months Ended
May 23, 2019
Jun. 30, 2019
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2019
Jun. 30, 2018
Dec. 31, 2018
Business Acquisition [Line Items]              
Goodwill   $ 1,913 $ 1,913   $ 1,913   $ 1,290
Revenues     5,092 $ 4,789 10,032 $ 9,480  
Net Income (Loss) Available to Common Stockholders, Basic     372 $ 582 997 $ 1,179  
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            
Goodwill 623            
Revenues   178          
Net Income (Loss) Available to Common Stockholders, Basic   $ 141          
Business Combination, Acquisition Related Costs         15    
The Navigators Group, Inc. [Member] | Cash [Member]              
Business Acquisition [Line Items]              
Payments to Acquire Businesses, Gross 2,098            
Adverse Development Cover Navigators Group [Member]              
Business Acquisition [Line Items]              
Reinsurance premium         91    
Reinsurance Retention Policy, Amount Retained         68    
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims     97        
Use of Retention Layer by Acquiree Prior to Acquisition $ 32            
Current accident year     29        
Policyholder Benefits and Claims Incurred, Ceded         91    
Adverse Development Cover Navigators Group [Member] | Remaining [Member]              
Business Acquisition [Line Items]              
Reinsurance Retention Policy, Excess Retention, Amount Reinsured         209    
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        
Adverse Development Cover Navigators Group [Member] | Maximum [Member]              
Business Acquisition [Line Items]              
Reinsurance Retention Policy, Excess Retention, Amount Reinsured         300    
Recorded by Acquiree Prior to Acquisition [Member]              
Business Acquisition [Line Items]              
Current accident year     $ 52        
Retention Layer Above Reserve for the Covered Liabilities as of the Inception Date [Member] | Adverse Development Cover Navigators Group [Member]              
Business Acquisition [Line Items]              
Reinsurance Retention Policy, Amount Retained         100    
Retention Layer for Reserve for the Covered Liabilities as of the Inception Date [Member] | Adverse Development Cover Navigators Group [Member]              
Business Acquisition [Line Items]              
Reinsurance Retention Policy, Amount Retained         $ 1,816