Accumulated other comprehensive income - Reclassified Amounts (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Investment gains/losses	$ (315)	$ (1,850)
Unrealized appreciation of investments, net [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassifications before income taxes	(305)	(1,807)
Applicable income taxes	(107)	(632)
Total amounts reclassified from other comprehensive income into net earnings	(198)	(1,175)
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	(305)	(1,807)
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	24	21
Applicable income taxes	6	5
Total amounts reclassified from other comprehensive income into net earnings	18	16
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	24	21
Other AOCI transactions [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassifications before income taxes	14	21
Applicable income taxes	6	9
Total amounts reclassified from other comprehensive income into net earnings	8	12
Other AOCI transactions [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other	14	21
Accumulated other comprehensive income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassifications before income taxes	(267)	(1,765)
Applicable income taxes	(95)	(618)
Total amounts reclassified from other comprehensive income into net earnings	(172)	(1,147)
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	(305)	(1,807)
Other	$ 38	$ 42