Accumulated other comprehensive income (Detail) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of year	$ 443,164
Other comprehensive income, net	24	$ (724)
Balance at end of period	470,409
Unrealized appreciation of investments, net [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of year	536	481
Other comprehensive income, net	(58)	14
Balance at end of period	478	495
Foreign currency translation [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of year	(3,082)	(4,346)
Other comprehensive income, net	(1)	(794)
Balance at end of period	(3,083)	(5,140)
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	73	83
Balance at end of period	(1,572)	(1,286)
Other AOCI transactions [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of year	(52)	(9)
Other comprehensive income, net	10	(27)
Balance at end of period	(42)	(36)
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	$ (4,219)	$ (5,967)