Reinsurance - Consolidated Property Casualty Insurance Assumed and Ceded Business (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Property Casualty Insurance Assumed And Ceded Business [Line Items]
Incurred loss and loss expenses	$ 879	$ 874	$ 2,616	$ 2,581
Consolidated Property and Casualty Insurance Entity
Property Casualty Insurance Assumed And Ceded Business [Line Items]
Direct written premiums	1,248	1,221	3,831	3,712
Assumed written premiums	42	26	143	101
Ceded written premiums	(44)	(39)	(121)	(103)
Net written premiums	1,246	1,208	3,853	3,710
Direct earned premiums	1,238	1,195	3,680	3,547
Assumed earned premiums	38	39	107	99
Ceded earned premiums	(39)	(43)	(120)	(123)
Earned premiums	1,237	1,191	3,667	3,523
Direct incurred loss and loss expenses	792	760	2,394	2,318
Assumed incurred loss and loss expenses	26	62	55	97
Ceded incurred loss and loss expenses	(5)	(7)	(24)	(18)
Incurred loss and loss expenses	$ 813	$ 815	$ 2,425	$ 2,397