Reinsurance - Consolidated Property Casualty Insurance Assumed and Ceded Business (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Property Casualty Insurance Assumed And Ceded Business [Line Items]
Incurred loss and loss expenses	$ 1,003	$ 860
Consolidated Property and Casualty Insurance Entity
Property Casualty Insurance Assumed And Ceded Business [Line Items]
Direct written premiums	1,458	1,335
Assumed written premiums	111	87
Ceded written premiums	(51)	(41)
Net written premiums	1,518	1,381
Direct earned premiums	1,371	1,266
Assumed earned premiums	66	43
Ceded earned premiums	(48)	(42)
Earned premiums	1,389	1,267
Direct incurred loss and loss expenses	900	787
Assumed incurred loss and loss expenses	34	25
Ceded incurred loss and loss expenses	(4)	(22)
Incurred loss and loss expenses	$ 930	$ 790