Accumulated other comprehensive income (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	$ 42,732	$ 44,025	$ 27,500
Other comprehensive income, net before reclassifications	(7,470)	802	18,044
Reclassifications from accumulated other comprehensive income	(1,280)	(2,095)	(1,498)
Transactions with noncontrolling interests			(21)
Ending Balance	33,982	42,732	44,025
Unrealized appreciation of investments, net [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	45,636	44,042	29,254
Other comprehensive income, net before reclassifications	(5,522)	3,778	16,379
Reclassifications from accumulated other comprehensive income	(1,516)	(2,184)	(1,591)
Ending Balance	38,598	45,636	44,042
Foreign currency translation [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(1,957)	(146)	(120)
Other comprehensive income, net before reclassifications	(2,027)	(1,877)	25
Reclassifications from accumulated other comprehensive income	128	66	(31)
Transactions with noncontrolling interests			(20)
Ending Balance	(3,856)	(1,957)	(146)
Prior service and actuarial gains/losses of defined benefit pension plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(1,039)	46	(1,601)
Other comprehensive income, net before reclassifications	191	(1,130)	1,534
Reclassifications from accumulated other comprehensive income	86	45	114
Transactions with noncontrolling interests			(1)
Ending Balance	(762)	(1,039)	46
Other AOCI transactions [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	92	83	(33)
Other comprehensive income, net before reclassifications	(112)	31	106
Reclassifications from accumulated other comprehensive income	22	(22)	10
Ending Balance	$ 2	$ 92	$ 83