Segment Information - Net Income (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Net income (loss)	$ (3,703)	$ 234	$ (40)	$ 378	$ (81)	$ 438	$ 216	$ 323	$ (3,131)	$ 896	$ 1,682
Net servicing income									85	86	87
Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Net income (loss)									865	994	991
Operating Segments | Personal Lines
Segment Reporting Information [Line Items]
Net income (loss)									(9)	(9)	199
Operating Segments | Property & Casualty Other Operations
Segment Reporting Information [Line Items]
Net income (loss)									69	(529)	(53)
Operating Segments | Group Benefits
Segment Reporting Information [Line Items]
Net income (loss)									294	230	187
Operating Segments | Mutual Funds
Segment Reporting Information [Line Items]
Net income (loss)									106	78	86
Corporate
Segment Reporting Information [Line Items]
Net income (loss)									$ (4,456)	132	272
Scenario, Adjustment | Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Net servicing income										(13)	(12)
Scenario, Adjustment | Operating Segments | Personal Lines
Segment Reporting Information [Line Items]
Net servicing income										$ 13	$ 12