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Reserve for Unpaid Losses and Loss Adjustment Expenses - Additional Information (Details) - USD ($)
$ in Millions
 3 Months Ended 12 Months Ended
Dec. 31, 2017
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Liability for Claims and Claims Adjustment Expense [Line Items]        
Current accident year benefit from discounting property and casualty reserves   $ 15 $ 27 $ 35
Property and Casualty Insurance Products        
Liability for Claims and Claims Adjustment Expense [Line Items]        
Prior accident year development   $ (41) $ 457 $ 250
Property and Casualty Insurance Products | Minimum        
Liability for Claims and Claims Adjustment Expense [Line Items]        
Range of discount rates   1.77% 1.77% 1.77%
Property and Casualty Insurance Products | Maximum        
Liability for Claims and Claims Adjustment Expense [Line Items]        
Range of discount rates   14.15% 14.15% 14.15%
Asbestos and Environmental        
Liability for Claims and Claims Adjustment Expense [Line Items]        
Adverse development from comprehensive annual review $ 285 $ 285    
Reinsurance recoverable during period   285    
Group Insurance Policies        
Liability for Claims and Claims Adjustment Expense [Line Items]        
Prior accident year development   $ (185) $ (162) $ (146)
Group Insurance Policies | Minimum        
Liability for Claims and Claims Adjustment Expense [Line Items]        
Range of discount rates   2.10% 3.00% 3.00%
Group Insurance Policies | Maximum        
Liability for Claims and Claims Adjustment Expense [Line Items]        
Range of discount rates   8.00% 8.00% 8.00%
Group Insurance Policies | Group long-term disability        
Liability for Claims and Claims Adjustment Expense [Line Items]        
Prior accident year development   $ (125) $ 90 $ 90
Group Insurance Policies | Group life and accident, excluding premium waiver        
Liability for Claims and Claims Adjustment Expense [Line Items]        
Prior accident year development   $ 60 $ 75 $ 50