Investments in equity securities (Parenthetical) (Detail) $ in Millions	9 Months Ended		12 Months Ended
	Sep. 30, 2019 USD ($) Company		Dec. 31, 2018 USD ($) Company
Summary of Investment Holdings [Line Items]
Fair value of investments	$ 220,051	[1]	$ 172,757	[2]
American Express Company [Member]
Summary of Investment Holdings [Line Items]
Fair value of investments	17,900		14,500
Apple Inc. [Member]
Summary of Investment Holdings [Line Items]
Fair value of investments	57,000		40,300
Bank of America Corporation [Member]
Summary of Investment Holdings [Line Items]
Fair value of investments	27,800		22,600
The Coca-Cola Company [Member]
Summary of Investment Holdings [Line Items]
Fair value of investments	21,800		18,900
Wells Fargo & Company [Member]
Summary of Investment Holdings [Line Items]
Fair value of investments	$ 20,200		$ 20,700
Equity Securities [Member] | Investment Concentration [Member]
Summary of Investment Holdings [Line Items]
Concentration percentage	66.00%		68.00%
Number of companies in concentration percentage | Company	5		5

[1] Approximately 66% of the aggregate fair value was concentrated in five companies (American Express Company – $17.9 billion; Apple Inc. – $57.0 billion; Bank of America Corporation – $27.8 billion; The Coca-Cola Company – $21.8 billion and Wells Fargo & Company – $20.2 billion).

[2] Approximately 68% of the aggregate fair value was concentrated in five companies (American Express Company – $14.5 billion; Apple Inc. – $40.3 billion; Bank of America Corporation – $22.6 billion; The Coca-Cola Company – $18.9 billion and Wells Fargo & Company – $20.7 billion).