Investment gains/losses (Detail) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018
Gain (Loss) on Securities [Line Items]
Equity securities - Unrealized investment gains (losses) on securities held at the end of the period	$ 19,393	$ (7,807)
Equity securities - Investment gains (losses) during the period	161	(240)
Equity securities - Investment gains (losses), total	19,554	(8,047)
Fixed maturity securities - Gross realized gains	5	359
Fixed maturity securities - Gross realized losses	(10)	(138)
Investment gains (losses)	19,552	(7,809)
Other [Member]
Gain (Loss) on Securities [Line Items]
Investment gains (losses)	$ 3	$ 17

Amount of realized gain on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of realized and unrealized gain (loss) on investment.

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