Segment Information - Amortization of Deferred Policy Acquisition Costs (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Amortization of deferred policy acquisition costs (DAC)	$ 1,372	$ 1,377	$ 1,364
Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Amortization of deferred policy acquisition costs (DAC)	1,009	973	951
Operating Segments | Personal Lines
Segment Reporting Information [Line Items]
Amortization of deferred policy acquisition costs (DAC)	309	348	359
Operating Segments | Group Benefits
Segment Reporting Information [Line Items]
Amortization of deferred policy acquisition costs (DAC)	33	31	31
Operating Segments | Mutual Funds
Segment Reporting Information [Line Items]
Amortization of deferred policy acquisition costs (DAC)	21	24	22
Corporate
Segment Reporting Information [Line Items]
Amortization of deferred policy acquisition costs (DAC)	$ 0	$ 1	$ 1