Insurance - Reinsurance information table including assumed, ceded, and recoveries (Details) - Property and casualty insurance - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Effects of Reinsurance [Line Items]
Direct premiums written	$ 6,862	$ 7,044	$ 6,626
Reinsurance assumed	225	255	214
Reinsurance ceded	(2,074)	(1,957)	(1,817)
Net written premiums	5,013	5,342	5,023
Direct premiums earned	6,846	6,848	6,472
Reinsurance assumed	237	226	204
Reinsurance ceded	(1,984)	(1,889)	(1,811)
Net earned premiums	5,099	5,185	4,865
Reinsurance recoveries	$ 1,522	$ 1,404	$ 1,249