Insurance - Reinsurance information table including assumed, ceded, and recoveries (Details) - Property and casualty insurance - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Effects of Reinsurance [Line Items]
Direct premiums written	$ 7,044	$ 6,626	$ 6,310
Reinsurance assumed	255	214	192
Reinsurance ceded	(1,957)	(1,817)	(1,751)
Net written premiums	5,342	5,023	4,751
Direct premiums earned	6,848	6,472	6,112
Reinsurance assumed	226	204	157
Reinsurance ceded	(1,889)	(1,811)	(1,690)
Net earned premiums	5,185	4,865	4,579
Reinsurance recoveries	$ 1,404	$ 1,249	$ 1,379