Investments in equity securities (Parenthetical) (Detail) $ in Millions	Sep. 30, 2015 USD ($) Company		Dec. 31, 2014 USD ($) Company
Summary of Investment Holdings [Line Items]
Fair value of investments	$ 110,288	[1]	$ 117,470	[2]
American Express Company [Member]
Summary of Investment Holdings [Line Items]
Fair value of investments	11,200		14,100
Wells Fargo & Company [Member]
Summary of Investment Holdings [Line Items]
Fair value of investments	25,200		26,500
International Business Machines Corporation [Member]
Summary of Investment Holdings [Line Items]
Fair value of investments	11,700		12,300
The Coca-Cola Company [Member]
Summary of Investment Holdings [Line Items]
Fair value of investments	$ 16,000		$ 16,900
Equity Securities [Member] | Investment Concentration [Member]
Summary of Investment Holdings [Line Items]
Concentration percentage	58.00%		59.00%
Number of companies in concentration percentage | Company	4		4

[1] Approximately 58% of the aggregate fair value was concentrated in the equity securities of four companies (American Express Company - $11.2 billion; Wells Fargo & Company - $25.2 billion; International Business Machines Corporation ("IBM") - $11.7 billion; and The Coca-Cola Company - $16.0 billion).

[2] Approximately 59% of the aggregate fair value was concentrated in the equity securities of four companies (American Express Company - $14.1 billion; Wells Fargo & Company - $26.5 billion; International Business Machines Corporation ("IBM") - $12.3 billion; and The Coca-Cola Company - $16.9 billion).