Reinsurance - Written Consolidated Property Casualty Insurance Premium on Assumed and Ceded Business (Details) - Consolidated Property and Casualty Insurance Entity - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Property Casualty Insurance Premiums Written On Assumed And Ceded Business [Line Items]
Direct written premiums	$ 1,221	$ 1,189	$ 3,712	$ 3,561
Assumed written premiums	26	23	101	86
Ceded written premiums	(39)	(37)	(103)	(131)
Net written premiums	$ 1,208	$ 1,175	$ 3,710	$ 3,516