Investments in equity securities (Parenthetical) (Detail) $ in Millions	Mar. 31, 2018 USD ($) Company		Dec. 31, 2017 USD ($) Company
Summary of Investment Holdings [Line Items]
Fair value of investments	$ 172,976	[1]	$ 170,540	[2]
Equity Securities [Member] | Investment Concentration [Member]
Summary of Investment Holdings [Line Items]
Concentration percentage	68.00%		65.00%
Number of companies in concentration percentage | Company	5		5
American Express Company [Member]
Summary of Investment Holdings [Line Items]
Fair value of investments	$ 14,100		$ 15,100
Apple Inc. [Member]
Summary of Investment Holdings [Line Items]
Fair value of investments	40,700		28,200
Bank of America Corporation [Member]
Summary of Investment Holdings [Line Items]
Fair value of investments	21,000		20,700
The Coca-Cola Company [Member]
Summary of Investment Holdings [Line Items]
Fair value of investments	17,400		18,400
Wells Fargo & Company [Member]
Summary of Investment Holdings [Line Items]
Fair value of investments	$ 25,200		$ 29,300

[1] Approximately 68% of the aggregate fair value was concentrated in five companies (American Express Company - $14.1 billion; Apple Inc. - $40.7 billion; Bank of America Corporation - $21.0 billion; The Coca-Cola Company - $17.4 billion and Wells Fargo & Company - $25.2 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).