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Segment Information - Net Investment Income (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Segment Reporting Information [Line Items]      
Net investment income $ 1,603 $ 1,577 $ 1,561
Operating Segments | Commercial Lines      
Segment Reporting Information [Line Items]      
Net investment income 949 917 910
Operating Segments | Personal Lines      
Segment Reporting Information [Line Items]      
Net investment income 141 135 128
Operating Segments | Property & Casualty Other Operations      
Segment Reporting Information [Line Items]      
Net investment income 106 127 133
Operating Segments | Group Benefits      
Segment Reporting Information [Line Items]      
Net investment income 381 366 371
Operating Segments | Mutual Funds      
Segment Reporting Information [Line Items]      
Net investment income 3 1 1
Corporate      
Segment Reporting Information [Line Items]      
Net investment income $ 23 $ 31 $ 18