Accumulated other comprehensive income - Reclassified Amounts (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Investment gains/losses	$ (290)	$ (643)	$ (605)	$ (2,493)
Unrealized appreciation of investments, net [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassifications before income taxes			(589)	(1,816)
Applicable income taxes			(206)	(636)
Total reclassifications into net earnings			(383)	(1,180)
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			(589)	(1,816)
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			45	51
Applicable income taxes			11	16
Total reclassifications into net earnings			34	35
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			45	51
Other AOCI transactions [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassifications before income taxes			24	35
Applicable income taxes			11	19
Total reclassifications into net earnings			13	16
Other AOCI transactions [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other			24	35
Accumulated other comprehensive income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassifications before income taxes			(520)	(1,730)
Applicable income taxes			(184)	(601)
Total reclassifications into net earnings			(336)	(1,129)
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			(589)	(1,816)
Other			$ 69	$ 86