Segment Information - Net Income (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Segment Reporting Information [Line Items]
Net Income (Loss) Attributable to Parent	$ 597	$ 378
Operating Segments | Commercial Lines Segment [Member]
Segment Reporting Information [Line Items]
Net Income (Loss) Attributable to Parent	298	231
Operating Segments | Personal Lines Segment [Member]
Segment Reporting Information [Line Items]
Net Income (Loss) Attributable to Parent	89	33
Operating Segments | Property & Casualty Other Operations
Segment Reporting Information [Line Items]
Net Income (Loss) Attributable to Parent	17	24
Operating Segments | Group Benefits Segment [Member]
Segment Reporting Information [Line Items]
Net Income (Loss) Attributable to Parent	54	45
Operating Segments | Mutual Funds
Segment Reporting Information [Line Items]
Net Income (Loss) Attributable to Parent	34	23
Corporate
Segment Reporting Information [Line Items]
Net Income (Loss) Attributable to Parent	$ 105	$ 22