Investments in equity securities (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	0 Months Ended
	Sep. 30, 2014 Item		Dec. 31, 2013 Item
Summary of Investment Holdings [Line Items]
Fair value of investments	$ 118,909	[1]	$ 117,505	[2]
American Express Company [Member]
Summary of Investment Holdings [Line Items]
Fair value of investments	13,300		13,800
Wells Fargo & Company [Member]
Summary of Investment Holdings [Line Items]
Fair value of investments	25,100		21,900
International Business Machines Corporation [Member]
Summary of Investment Holdings [Line Items]
Fair value of investments	13,400		12,800
The Coca-Cola Company [Member]
Summary of Investment Holdings [Line Items]
Fair value of investments	$ 17,100		$ 16,500
Investment Concentration [Member] | Equity Securities [Member]
Summary of Investment Holdings [Line Items]
Concentration percentage	58.00%		55.00%
Number of companies in concentration percentage	4		4

[1] As of September 30, 2014, approximately 58% of the aggregate fair value was concentrated in the equity securities of four companies (American Express Company - $13.3 billion; Wells Fargo & Company - $25.1 billion; International Business Machines Corporation - $13.4 billion; and The Coca-Cola Company - $17.1 billion).

[2] As of December 31, 2013, approximately 55% of the aggregate fair value was concentrated in the equity securities of four companies (American Express Company - $13.8 billion; Wells Fargo & Company - $21.9 billion; International Business Machines Corporation - $12.8 billion; and The Coca-Cola Company - $16.5 billion).