Insurance - Reinsurance information table including assumed, ceded, and recoveries (Details) - Property and casualty insurance - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Effects of Reinsurance [Line Items]
Direct premiums written	$ 6,626	$ 6,310	$ 5,858
Reinsurance assumed	214	192	123
Reinsurance ceded	(1,817)	(1,751)	(1,595)
Net written premiums	5,023	4,751	4,386
Direct premiums earned	6,472	6,112	5,745
Reinsurance assumed	204	157	118
Reinsurance ceded	(1,811)	(1,690)	(1,535)
Net earned premiums	4,865	4,579	4,328
Reinsurance recoveries	$ 1,249	$ 1,379	$ 810