Business segment data - Insurance premiums by type (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Property/Casualty [Member]
Segment Reporting Information [Line Items]
Premiums written - Direct	$ 47,838	$ 47,578	$ 44,513
Premiums written - Assumed	11,533	10,214	8,970
Premiums written - Ceded	(898)	(821)	(869)
Premiums written	58,473	56,971	52,614
Premiums earned - Direct	46,418	46,540	43,095
Premiums earned - Assumed	11,449	9,643	8,649
Premiums earned - Ceded	(907)	(851)	(825)
Premiums earned	56,960	55,332	50,919
Life/Health [Member]
Segment Reporting Information [Line Items]
Premiums written - Direct	510	839	1,111
Premiums written - Assumed	5,960	5,046	5,540
Premiums written - Ceded	(42)	(45)	(49)
Premiums written	6,428	5,840	6,602
Premiums earned - Direct	510	839	1,111
Premiums earned - Assumed	5,973	4,952	5,438
Premiums earned - Ceded	(42)	(45)	(50)
Premiums earned	$ 6,441	$ 5,746	$ 6,499