Investments in equity securities (Parenthetical) (Detail) $ in Millions	Sep. 30, 2018 USD ($) Company		Dec. 31, 2017 USD ($) Company
Summary of Investment Holdings [Line Items]
Fair value of investments	$ 207,332	[1]	$ 170,540	[2]
American Express Company [Member]
Summary of Investment Holdings [Line Items]
Fair value of investments	16,100		15,100
Apple Inc. [Member]
Summary of Investment Holdings [Line Items]
Fair value of investments	57,600		28,200
Bank of America Corporation [Member]
Summary of Investment Holdings [Line Items]
Fair value of investments	26,500		20,700
The Coca-Cola Company [Member]
Summary of Investment Holdings [Line Items]
Fair value of investments	18,500		18,400
Wells Fargo & Company [Member]
Summary of Investment Holdings [Line Items]
Fair value of investments	$ 24,400		$ 29,300
Equity Securities [Member] | Investment Concentration [Member]
Summary of Investment Holdings [Line Items]
Concentration percentage	69.00%		65.00%
Number of companies in concentration percentage | Company	5		5

[1] Approximately 69% of the aggregate fair value was concentrated in five companies (American Express Company - $16.1 billion; Apple Inc. - $57.6 billion; Bank of America Corporation - $26.5 billion; The Coca-Cola Company - $18.5 billion and Wells Fargo & Company - $24.4 billion).

[2] Approximately 65% of the aggregate fair value was concentrated in five companies (American Express Company - $15.1 billion; Apple Inc. - $28.2 billion; Bank of America Corporation - $20.7 billion; The Coca-Cola Company - $18.4 billion and Wells Fargo & Company - $29.3 billion).