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		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Property Casualty Insurance Premiums Written On Assumed And Ceded Business [Line Items]
Direct written premiums	$ 1,265	$ 1,211	$ 2,491	$ 2,372
Assumed written premiums	42	42	75	63
Ceded written premiums	(36)	(59)	(64)	(94)
Net written premiums	$ 1,271	$ 1,194	$ 2,502	$ 2,341