Reinsurance - Consolidated Property Casualty Insurance Assumed and Ceded Business (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Property Casualty Insurance Assumed And Ceded Business [Line Items]
Incurred loss and loss expenses	$ 883	$ 854	$ 1,737	$ 1,707
Consolidated Property and Casualty Insurance Entity
Property Casualty Insurance Assumed And Ceded Business [Line Items]
Direct written premiums	1,336	1,265	2,583	2,491
Assumed written premiums	52	42	101	75
Ceded written premiums	(39)	(36)	(77)	(64)
Net written premiums	1,349	1,271	2,607	2,502
Direct earned premiums	1,235	1,189	2,442	2,352
Assumed earned premiums	36	33	69	60
Ceded earned premiums	(41)	(41)	(81)	(80)
Earned premiums	1,230	1,181	2,430	2,332
Direct incurred loss and loss expenses	821	770	1,602	1,558
Assumed incurred loss and loss expenses	13	20	29	35
Ceded incurred loss and loss expenses	(13)	4	(19)	(11)
Incurred loss and loss expenses	$ 821	$ 794	$ 1,612	$ 1,582