Investments in equity securities (Parenthetical) (Detail) $ in Millions	Dec. 31, 2017 USD ($) Company		Dec. 31, 2016 USD ($) Company
Summary of Investment Holdings [Line Items]
Fair value of investments	$ 170,540	[1]	$ 139,288	[2]
Equity Securities [Member] | Investment Concentration [Member]
Summary of Investment Holdings [Line Items]
Concentration percentage	65.00%		60.00%
Number of companies in concentration percentage | Company	5		5
American Express Company [Member]
Summary of Investment Holdings [Line Items]
Fair value of investments	$ 15,100		$ 11,200
Apple Inc. [Member]
Summary of Investment Holdings [Line Items]
Fair value of investments	28,200
Bank of America Corporation [Member]
Summary of Investment Holdings [Line Items]
Fair value of investments	20,700		14,500
The Coca-Cola Company [Member]
Summary of Investment Holdings [Line Items]
Fair value of investments	18,400		16,600
Wells Fargo & Company [Member]
Summary of Investment Holdings [Line Items]
Fair value of investments	$ 29,300		27,600
IBM [Member]
Summary of Investment Holdings [Line Items]
Fair value of investments			$ 13,500

[1] 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).

[2] Approximately 60% of the aggregate fair value was concentrated in five companies (American Express Company - $11.2 billion; Bank of America Corporation - $14.5 billion; The Coca-Cola Company - $16.6 billion; International Business Machines Corporation - $13.5 billion and Wells Fargo & Company - $27.6 billion).