Segment Information - Net Income (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Segment Reporting Information [Line Items]
Net income	$ 432	$ 234	$ 1,611	$ 572
Corporate
Segment Reporting Information [Line Items]
Net income	(35)	21	136	(392)
Property and Casualty, Commercial Insurance Product Line [Member]
Segment Reporting Information [Line Items]
Net income	289	90	959	579
Property and Casualty, Personal Insurance Product Line [Member]
Segment Reporting Information [Line Items]
Net income	51	8	146	65
Property & Casualty Other Operations
Segment Reporting Information [Line Items]
Net income	9	18	31	62
Group Insurance Policies [Member]
Segment Reporting Information [Line Items]
Net income	77	71	227	185
Mutual Fund [Member]
Segment Reporting Information [Line Items]
Net income	$ 41	$ 26	$ 112	$ 73