Accumulated other comprehensive income (Detail) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of year	$ 443,164
Other comprehensive income, net	(591)	$ 53
Balance at end of period	472,480
Unrealized appreciation of investments, net [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of year	536	481
Other comprehensive income, net	(126)	29
Balance at end of period	410	510
Foreign currency translation [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of year	(3,082)	(4,346)
Other comprehensive income, net	(619)	(28)
Balance at end of period	(3,701)	(4,374)
Defined benefit pension plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of year	(1,645)	(1,369)
Other comprehensive income, net	112	67
Balance at end of period	(1,533)	(1,302)
Other AOCI transactions [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of year	(52)	(9)
Other comprehensive income, net	42	(15)
Balance at end of period	(10)	(24)
Accumulated other comprehensive income [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of year	(4,243)	(5,243)
Balance at end of period	$ (4,834)	$ (5,190)