Accumulated other comprehensive income (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	$ 352,500	$ 351,954
Ending Balance	372,591	351,052
Unrealized appreciation of investments, net [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	370	62,093
Reclassifications to retained earnings upon adoption of new accounting standards		(61,340)
Other comprehensive income, net before reclassifications	89	(74)
Reclassifications before income taxes	5	(221)
Applicable income taxes	(1)	46
Ending Balance	463	504
Foreign currency translation [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(4,603)	(3,114)
Reclassifications to retained earnings upon adoption of new accounting standards		(65)
Other comprehensive income, net before reclassifications	157	585
Ending Balance	(4,446)	(2,594)
Defined benefit pension plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(816)	(420)
Reclassifications to retained earnings upon adoption of new accounting standards		36
Other comprehensive income, net before reclassifications	38	(17)
Reclassifications before income taxes	16	10
Applicable income taxes	(4)	(1)
Ending Balance	(766)	(392)
Other AOCI transactions [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	34	12
Reclassifications to retained earnings upon adoption of new accounting standards		(6)
Other comprehensive income, net before reclassifications	(13)	2
Reclassifications before income taxes	2	(5)
Applicable income taxes	(1)	2
Ending Balance	22	5
Accumulated other comprehensive income [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(5,015)	58,571
Reclassifications to retained earnings upon adoption of new accounting standards		(61,375)
Other comprehensive income, net before reclassifications	271	496
Reclassifications before income taxes	23	(216)
Applicable income taxes	(6)	47
Ending Balance	$ (4,727)	$ (2,477)