Accumulated other comprehensive income - Reclassified Amounts (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Investment gains/losses	$ (657)	$ (3,150)	$ (1,262)	$ (5,643)
Unrealized appreciation of investments, net [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassifications before income taxes			(1,235)	(4,904)
Applicable income taxes			(432)	(1,716)
Total reclassifications into net earnings			(803)	(3,188)
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			(1,235)	(4,904)
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			82	79
Applicable income taxes			21	20
Total reclassifications into net earnings			61	59
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			82	79
Other AOCI transactions [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassifications before income taxes			32	41
Applicable income taxes			14	18
Total reclassifications into net earnings			18	23
Other AOCI transactions [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other			32	41
Accumulated other comprehensive income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassifications before income taxes			(1,121)	(4,784)
Applicable income taxes			(397)	(1,678)
Total reclassifications into net earnings			(724)	(3,106)
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			(1,235)	(4,904)
Other			$ 114	$ 120