Investments in equity securities (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2014 Item		Dec. 31, 2013 Item
Summary of Investment Holdings [Line Items]
Fair value of investments	$ 118,486		$ 117,505
Equity Securities [Member]
Summary of Investment Holdings [Line Items]
Fair value of investments	118,486	[1]	117,505	[2]
Investment Concentration [Member] | Equity Securities [Member]
Summary of Investment Holdings [Line Items]
Concentration percentage	56.00%		55.00%
Number of companies in concentration percentage	4		4
American Express Company [Member]
Summary of Investment Holdings [Line Items]
Fair value of investments	13,600		13,800
Wells Fargo & Company [Member]
Summary of Investment Holdings [Line Items]
Fair value of investments	24,000		21,900
International Business Machines Corporation [Member]
Summary of Investment Holdings [Line Items]
Fair value of investments	13,200		12,800
The Coca-Cola Company [Member]
Summary of Investment Holdings [Line Items]
Fair value of investments	$ 15,500		$ 16,500

[1] As of March 31, 2014, approximately 56% of the aggregate fair value was concentrated in the equity securities of four companies (American Express Company - $13.6 billion; Wells Fargo & Company - $24.0 billion; International Business Machines Corporation - $13.2 billion; and The Coca-Cola Company - $15.5 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).