Investments in equity securities (Parenthetical) (Detail) $ in Millions	Mar. 31, 2016 USD ($) Company		Dec. 31, 2015 USD ($) Company
Summary of Investment Holdings [Line Items]
Fair value of investments	$ 106,420	[1]	$ 111,822	[2]
American Express Company [Member]
Summary of Investment Holdings [Line Items]
Fair value of investments	9,300		10,500
Wells Fargo & Company [Member]
Summary of Investment Holdings [Line Items]
Fair value of investments	24,200		27,200
International Business Machines Corporation [Member]
Summary of Investment Holdings [Line Items]
Fair value of investments	12,300		11,200
The Coca-Cola Company [Member]
Summary of Investment Holdings [Line Items]
Fair value of investments	$ 18,600		$ 17,200
Equity Securities [Member] | Investment Concentration [Member]
Summary of Investment Holdings [Line Items]
Concentration percentage	60.00%		59.00%
Number of companies in concentration percentage | Company	4		4

[1] Approximately 60% of the aggregate fair value was concentrated in the equity securities of four companies (American Express Company - $9.3 billion; Wells Fargo & Company - $24.2 billion; International Business Machines Corporation ("IBM") - $12.3 billion; and The Coca-Cola Company - $18.6 billion).

[2] Approximately 59% of the aggregate fair value was concentrated in the equity securities of four companies (American Express Company - $10.5 billion; Wells Fargo & Company - $27.2 billion; International Business Machines Corporation - $11.2 billion; and The Coca-Cola Company - $17.2 billion).