Reinsurance - Consolidated Property Casualty Insurance Assumed and Ceded Business (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Property Casualty Insurance Assumed And Ceded Business [Line Items]
Incurred loss and loss expenses	$ 936	$ 883	$ 1,796	$ 1,737
Consolidated Property and Casualty Insurance Entity
Property Casualty Insurance Assumed And Ceded Business [Line Items]
Direct written premiums	1,472	1,336	2,807	2,583
Assumed written premiums	75	52	162	101
Ceded written premiums	(71)	(39)	(112)	(77)
Net written premiums	1,476	1,349	2,857	2,607
Direct earned premiums	1,319	1,235	2,585	2,442
Assumed earned premiums	50	36	93	69
Ceded earned premiums	(52)	(41)	(94)	(81)
Earned premiums	1,317	1,230	2,584	2,430
Direct incurred loss and loss expenses	866	821	1,653	1,602
Assumed incurred loss and loss expenses	25	13	50	29
Ceded incurred loss and loss expenses	(28)	(13)	(50)	(19)
Incurred loss and loss expenses	$ 863	$ 821	$ 1,653	$ 1,612