Segment Information - Assets (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Total assets	$ 225,260	$ 224,576
Reinsurance recoverables, net	4,061	3,659
Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Total assets	31,281	29,845
Operating Segments | Personal Lines
Segment Reporting Information [Line Items]
Total assets	6,251	6,091
Operating Segments | Property & Casualty Other Operations
Segment Reporting Information [Line Items]
Total assets	3,568	4,732
Operating Segments | Group Benefits
Segment Reporting Information [Line Items]
Total assets	14,478	8,825
Operating Segments | Mutual Funds
Segment Reporting Information [Line Items]
Total assets	547	480
Corporate
Segment Reporting Information [Line Items]
Total assets	169,135	174,603
Reinsurance recoverables, net	349	366
Scenario, Adjustment | Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Total assets		(8)
Scenario, Adjustment | Operating Segments | Personal Lines
Segment Reporting Information [Line Items]
Total assets		8
Property and Casualty Insurance Products | Operating Segments
Segment Reporting Information [Line Items]
Reinsurance recoverables, net	3,476	3,085
Property and Casualty Insurance Products | Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Liability for unpaid losses and loss adjustment expense, held for sale	688	712	$ 743
Reinsurance recoverables, net	$ 688	$ 712