Accumulated other comprehensive income - Reclassified Amounts (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Investment gains/losses	$ (7,553)	$ (9,373)	$ (3,575)
Unrealized appreciation of investments, net [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassifications before income taxes	(6,820)	(2,332)	(3,360)
Applicable income taxes	(2,387)	(816)	(1,176)
Total amounts reclassified from other comprehensive income into net earnings	(4,433)	(1,516)	(2,184)
Unrealized appreciation of investments, net [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Investment gains/losses	(6,820)	(2,332)	(3,360)
Foreign currency translation [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassifications before income taxes		197	75
Applicable income taxes		69	9
Total amounts reclassified from other comprehensive income into net earnings		128	66
Foreign currency translation [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Investment gains/losses		197
Other			75
Prior service and actuarial gains/losses of defined benefit pension plans [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassifications before income taxes	104	129	58
Applicable income taxes	29	43	13
Total amounts reclassified from other comprehensive income into net earnings	75	86	45
Prior service and actuarial gains/losses of defined benefit pension plans [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other	104	129	58
Other AOCI transactions [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassifications before income taxes	51	35	(39)
Applicable income taxes	22	13	(17)
Total amounts reclassified from other comprehensive income into net earnings	29	22	(22)
Other AOCI transactions [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other	51	35	(39)
Accumulated other comprehensive income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassifications before income taxes	(6,665)	(1,971)	(3,266)
Applicable income taxes	(2,336)	(691)	(1,171)
Total amounts reclassified from other comprehensive income into net earnings	(4,329)	(1,280)	(2,095)
Accumulated other comprehensive income [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Investment gains/losses	(6,820)	(2,135)	(3,360)
Other	$ 155	$ 164	$ 94