Accumulated other comprehensive income (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	$ 424,791
Other comprehensive income, net	1,000	$ (228)	$ (2,211)
Ending Balance	443,164	424,791
Cumulative effect, period of adoption, adjustment [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance			(61,375)
Unrealized appreciation of investments, net [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	481	370	62,093
Other comprehensive income, net	55	111	(383)
Ending Balance	536	481	370
Unrealized appreciation of investments, net [Member] | Cumulative effect, period of adoption, adjustment [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance			(61,340)
Foreign currency translation [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(4,346)	(4,603)	(3,114)
Other comprehensive income, net	1,264	257	(1,424)
Ending Balance	(3,082)	(4,346)	(4,603)
Foreign currency translation [Member] | Cumulative effect, period of adoption, adjustment [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance			(65)
Defined benefit pension plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(1,369)	(816)	(420)
Other comprehensive income, net	(276)	(553)	(432)
Ending Balance	(1,645)	(1,369)	(816)
Defined benefit pension plans [Member] | Cumulative effect, period of adoption, adjustment [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance			36
Other AOCI transactions [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(9)	34	12
Other comprehensive income, net	(43)	(43)	28
Ending Balance	(52)	(9)	34
Other AOCI transactions [Member] | Cumulative effect, period of adoption, adjustment [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance			(6)
Accumulated other comprehensive income [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(5,243)	(5,015)	58,571
Ending Balance	$ (4,243)	$ (5,243)	$ (5,015)