Reinsurance - Effects of reinsurance on property and casualty premiums (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property and casualty insurance premiums earned
Direct	$ 35,115	$ 34,015	$ 32,915
Assumed	837	863	890
Ceded	(1,274)	(1,296)	(1,338)
Net amount	34,678	33,582	32,467
Life premiums and contract charges
Life premiums and contract charges, net of reinsurance	2,378	2,275	2,158
Property-liability insurance premiums earned
Property and casualty insurance premiums written
Direct	33,685	32,614	31,924
Assumed	64	47	39
Ceded	(1,007)	(1,061)	(1,092)
Property and casualty insurance premiums written, net of reinsurance	32,742	31,600	30,871
Property and casualty insurance premiums earned
Direct	33,221	32,249	31,274
Assumed	50	45	41
Ceded	(971)	(987)	(1,006)
Net amount	32,300	31,307	30,309
Life and annuity premiums and contract charges
Life premiums and contract charges
Direct	1,894	1,766	1,641
Assumed	787	818	849
Ceded	(303)	(309)	(332)
Life premiums and contract charges, net of reinsurance	$ 2,378	$ 2,275	$ 2,158