Business segment data - Insurance premiums by geographic area (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property/Casualty [Member]
Segment Reporting Information [Line Items]
Premiums written	$ 56,498	$ 41,240	$ 36,716
Property/Casualty [Member] | United States [Member]
Segment Reporting Information [Line Items]
Premiums written	50,604	35,878	31,171
Property/Casualty [Member] | Asia Pacific [Member]
Segment Reporting Information [Line Items]
Premiums written	3,307	3,616	3,472
Property/Casualty [Member] | Western Europe [Member]
Segment Reporting Information [Line Items]
Premiums written	1,516	1,406	1,638
Property/Casualty [Member] | All Other Geographic Areas [Member]
Segment Reporting Information [Line Items]
Premiums written	1,071	340	435
Life/Health [Member]
Segment Reporting Information [Line Items]
Premiums written	5,744	5,670	5,951
Life/Health [Member] | United States [Member]
Segment Reporting Information [Line Items]
Premiums written	3,320	3,473	3,247
Life/Health [Member] | Asia Pacific [Member]
Segment Reporting Information [Line Items]
Premiums written	879	715	673
Life/Health [Member] | Western Europe [Member]
Segment Reporting Information [Line Items]
Premiums written	909	822	1,263
Life/Health [Member] | All Other Geographic Areas [Member]
Segment Reporting Information [Line Items]
Premiums written	$ 636	$ 660	$ 768