Accumulated other comprehensive income - Reclassified Amounts (Detail) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Unrealized appreciation of investments, net [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassifications before income taxes	$ (299)	$ (1,235)
Applicable income taxes	(63)	(432)
Total reclassifications into net earnings	(236)	(803)
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	101	82
Applicable income taxes	25	21
Total reclassifications into net earnings	76	61
Other AOCI transactions [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassifications before income taxes	10	32
Applicable income taxes	3	14
Total reclassifications into net earnings	7	18
Accumulated other comprehensive income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassifications before income taxes	(188)	(1,121)
Applicable income taxes	(35)	(397)
Total reclassifications into net earnings	$ (153)	$ (724)