Accumulated other comprehensive income - Reclassified Amounts (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Investment gains/losses	$ (1,410)	$ (7,553)	$ (9,373)
Unrealized appreciation of investments, net [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassifications before income taxes	(1,399)	(6,820)	(2,332)
Applicable income taxes	(490)	(2,387)	(816)
Total reclassifications into net earnings	(909)	(4,433)	(1,516)
Foreign currency translation [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassifications before income taxes	3		197
Applicable income taxes			69
Total reclassifications into net earnings	3		128
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	155	104	129
Applicable income taxes	47	29	43
Total reclassifications into net earnings	108	75	86
Other AOCI transactions [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassifications before income taxes	19	51	35
Applicable income taxes	6	22	13
Total reclassifications into net earnings	13	29	22
Accumulated other comprehensive income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassifications before income taxes	(1,222)	(6,665)	(1,971)
Applicable income taxes	(437)	(2,336)	(691)
Total reclassifications into net earnings	(785)	(4,329)	(1,280)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Unrealized appreciation of investments, net [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Investment gains/losses	(1,399)	(6,820)	(2,332)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Foreign currency translation [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Investment gains/losses			197
Other	3
Reclassification out of Accumulated Other Comprehensive Income [Member] | Prior service and actuarial gains/losses of defined benefit pension plans [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other	155	104	129
Reclassification out of Accumulated Other Comprehensive Income [Member] | Other AOCI transactions [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other	19	51	35
Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated other comprehensive income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Investment gains/losses	(1,399)	(6,820)	(2,135)
Other	$ 177	$ 155	$ 164