Reinsurance - Consolidated Property Casualty Insurance Assumed and Ceded Business (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Property Casualty Insurance Assumed And Ceded Business [Line Items]
Incurred loss and loss expenses	$ 932	$ 879	$ 2,728	$ 2,616
Consolidated Property and Casualty Insurance Entity
Property Casualty Insurance Assumed And Ceded Business [Line Items]
Direct written premiums	1,361	1,248	4,168	3,831
Assumed written premiums	41	42	203	143
Ceded written premiums	(50)	(44)	(162)	(121)
Net written premiums	1,352	1,246	4,209	3,853
Direct earned premiums	1,386	1,238	3,971	3,680
Assumed earned premiums	52	38	145	107
Ceded earned premiums	(62)	(39)	(156)	(120)
Earned premiums	1,376	1,237	3,960	3,667
Direct incurred loss and loss expenses	855	792	2,508	2,394
Assumed incurred loss and loss expenses	37	26	87	55
Ceded incurred loss and loss expenses	(28)	(5)	(78)	(24)
Incurred loss and loss expenses	$ 864	$ 813	$ 2,517	$ 2,425