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Subsequent Events (Details)
$ in Millions
1 Months Ended 3 Months Ended 9 Months Ended
Oct. 31, 2017
USD ($)
segment
Dec. 31, 2017
USD ($)
Sep. 30, 2017
USD ($)
Sep. 30, 2016
USD ($)
Sep. 30, 2017
USD ($)
segment
Sep. 30, 2016
USD ($)
Jan. 03, 2017
USD ($)
Dec. 31, 2016
USD ($)
Subsequent Event [Line Items]                
Number of reportable segments | segment         4      
Goodwill     $ 2,309   $ 2,309     $ 1,219
Change in unrealized net capital gains and losses     125 $ 193 598 $ 1,197    
Subsequent Event                
Subsequent Event [Line Items]                
Number of reportable segments | segment 7              
Allstate Life | Subsequent Event                
Subsequent Event [Line Items]                
Goodwill $ 170              
Allstate Benefits | Subsequent Event                
Subsequent Event [Line Items]                
Goodwill 100              
Allstate Annuities | Subsequent Event                
Subsequent Event [Line Items]                
Goodwill $ 125              
Scenario, Forecast                
Subsequent Event [Line Items]                
Increase in future policy benefit reserves   $ 550            
Change in unrealized net capital gains and losses   (350)            
Scenario, Forecast | Allstate Annuities                
Subsequent Event [Line Items]                
Goodwill, impairment loss   $ 125            
SquareTrade                
Subsequent Event [Line Items]                
Goodwill     $ 1,090   $ 1,090   $ 1,080