Business segment data - Insurance premiums by geographic area (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Property/Casualty [Member]
Segment Reporting Information [Line Items]
Premiums written	$ 56,971	$ 52,614	$ 56,498
Life/Health [Member]
Segment Reporting Information [Line Items]
Premiums written	5,840	6,602	5,744
United States [Member] | Property/Casualty [Member]
Segment Reporting Information [Line Items]
Premiums written	50,529	46,146	50,604
United States [Member] | Life/Health [Member]
Segment Reporting Information [Line Items]
Premiums written	2,553	3,598	3,320
Asia Pacific [Member] | Property/Casualty [Member]
Segment Reporting Information [Line Items]
Premiums written	3,114	3,726	3,307
Asia Pacific [Member] | Life/Health [Member]
Segment Reporting Information [Line Items]
Premiums written	1,582	1,361	879
Western Europe [Member] | Property/Casualty [Member]
Segment Reporting Information [Line Items]
Premiums written	2,535	2,157	1,516
Western Europe [Member] | Life/Health [Member]
Segment Reporting Information [Line Items]
Premiums written	908	939	909
All Other [Member] | Property/Casualty [Member]
Segment Reporting Information [Line Items]
Premiums written	793	585	1,071
All Other [Member] | Life/Health [Member]
Segment Reporting Information [Line Items]
Premiums written	$ 797	$ 704	$ 636