Accumulated other comprehensive income (Detail) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of year	$ 424,791
Other comprehensive income, net	53	$ (393)
Balance at end of period	415,155
Unrealized appreciation of investments, net [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of year	481	370
Other comprehensive income, net	29	137
Balance at end of period	510	507
Foreign currency translation [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of year	(4,346)	(4,603)
Other comprehensive income, net	(28)	(534)
Balance at end of period	(4,374)	(5,137)
Defined benefit pension plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of year	(1,369)	(816)
Other comprehensive income, net	67	33
Balance at end of period	(1,302)	(783)
Other AOCI transactions [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of year	(9)	34
Other comprehensive income, net	(15)	(29)
Balance at end of period	(24)	5
Accumulated other comprehensive income [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of year	(5,243)	(5,015)
Balance at end of period	$ (5,190)	$ (5,408)