Business segment data - Insurance premiums by geographic area (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Property/Casualty [Member]
Segment Reporting Information [Line Items]
Premiums written	$ 52,614	$ 56,498	$ 41,240
Property/Casualty [Member] | United States [Member]
Segment Reporting Information [Line Items]
Premiums written	46,146	50,604	35,878
Property/Casualty [Member] | Asia Pacific [Member]
Segment Reporting Information [Line Items]
Premiums written	3,726	3,307	3,616
Property/Casualty [Member] | Western Europe [Member]
Segment Reporting Information [Line Items]
Premiums written	2,157	1,516	1,406
Property/Casualty [Member] | All Other Geographic Areas [Member]
Segment Reporting Information [Line Items]
Premiums written	585	1,071	340
Life/Health [Member]
Segment Reporting Information [Line Items]
Premiums written	6,602	5,744	5,670
Life/Health [Member] | United States [Member]
Segment Reporting Information [Line Items]
Premiums written	3,598	3,320	3,473
Life/Health [Member] | Asia Pacific [Member]
Segment Reporting Information [Line Items]
Premiums written	1,361	879	715
Life/Health [Member] | Western Europe [Member]
Segment Reporting Information [Line Items]
Premiums written	939	909	822
Life/Health [Member] | All Other Geographic Areas [Member]
Segment Reporting Information [Line Items]
Premiums written	$ 704	$ 636	$ 660