Segment Information - Net Income (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Segment Reporting Information [Line Items]
Net income (loss)	$ 582	$ (40)	$ 1,179	$ 338
Property and Casualty, Commercial Insurance Product Line [Member]
Segment Reporting Information [Line Items]
Net income (loss)	372	258	670	489
Property and Casualty, Personal Insurance Product Line [Member]
Segment Reporting Information [Line Items]
Net income (loss)	6	24	95	57
Property & Casualty Other Operations
Segment Reporting Information [Line Items]
Net income (loss)	5	20	22	44
Group Insurance Policies [Member]
Segment Reporting Information [Line Items]
Net income (loss)	96	69	150	114
Mutual Fund [Member]
Segment Reporting Information [Line Items]
Net income (loss)	37	24	71	47
Corporate
Segment Reporting Information [Line Items]
Net income (loss)	$ 66	$ (435)	$ 171	$ (413)