Insurance - Reinsurance information table including assumed, ceded, and recoveries (Details) - Property and casualty insurance - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Effects of Reinsurance [Line Items]
Direct premiums written	$ 5,858	$ 5,713	$ 5,387
Reinsurance assumed	123	119	90
Reinsurance ceded	(1,595)	(1,505)	(1,457)
Net written premiums	4,386	4,327	4,020
Direct premiums earned	5,745	5,613	5,195
Reinsurance assumed	118	105	75
Reinsurance ceded	(1,535)	(1,494)	(1,392)
Net earned premiums	4,328	4,224	3,878
Reinsurance recoveries	$ 810	$ 936	$ 895