Reinsurance - Consolidated Property Casualty Insurance Assumed and Ceded Business (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Property Casualty Insurance Assumed And Ceded Business [Line Items]
Incurred loss and loss expenses	$ 860	$ 854
Consolidated Property and Casualty Insurance Entity
Property Casualty Insurance Assumed And Ceded Business [Line Items]
Direct written premiums	1,335	1,247
Assumed written premiums	87	49
Ceded written premiums	(41)	(38)
Net written premiums	1,381	1,258
Direct earned premiums	1,266	1,207
Assumed earned premiums	43	33
Ceded earned premiums	(42)	(40)
Earned premiums	1,267	1,200
Direct incurred loss and loss expenses	787	781
Assumed incurred loss and loss expenses	25	16
Ceded incurred loss and loss expenses	(22)	(6)
Incurred loss and loss expenses	$ 790	$ 791