Segment Information - Income Tax Expense (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Income tax expense (benefit)	$ 985	$ (166)	$ 289
Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Income tax expense (benefit)	377	415	403
Operating Segments | Personal Lines
Segment Reporting Information [Line Items]
Income tax expense (benefit)	26	(23)	88
Operating Segments | Property & Casualty Other Operations
Segment Reporting Information [Line Items]
Income tax expense (benefit)	24	(355)	(47)
Operating Segments | Group Benefits
Segment Reporting Information [Line Items]
Income tax expense (benefit)	38	83	63
Operating Segments | Mutual Funds
Segment Reporting Information [Line Items]
Income tax expense (benefit)	63	43	48
Corporate
Segment Reporting Information [Line Items]
Income tax expense (benefit)	$ 457	$ (329)	$ (266)