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Quarterly Operating Results (Unaudited) - Narrative (Details) - USD ($)
$ in Millions
3 Months Ended 12 Months Ended
Dec. 31, 2019
Sep. 30, 2019
Sep. 30, 2018
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Quarterly Operating Results (Unaudited) [Line Items]            
Increase (decrease) in holding company expenses related to employee benefit plans $ 20          
Property and casualty insurance | Other lines            
Quarterly Operating Results (Unaudited) [Line Items]            
Increase in asbestos and environmental reserves   $ 18 $ 18 $ 18 $ 18 $ 89
Special charges related to the exit of certain lines of business within subsidiaries 76          
Other            
Quarterly Operating Results (Unaudited) [Line Items]            
Write off of deferred debt issuance cost due to the retirement of debt $ 5          
Other | Former railroad and manufacturing operations            
Quarterly Operating Results (Unaudited) [Line Items]            
Increase in asbestos and environmental reserves   $ 11 $ 9