Investments in equity securities (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	0 Months Ended
	Dec. 31, 2014 Item		Dec. 31, 2013 Item
Summary of Investment Holdings [Line Items]
Fair value of investments	117,470	[1]	117,505	[2]
American Express Company [Member]
Summary of Investment Holdings [Line Items]
Fair value of investments	14,100		13,800
Wells Fargo & Company [Member]
Summary of Investment Holdings [Line Items]
Fair value of investments	26,500		21,900
International Business Machines Corporation [Member]
Summary of Investment Holdings [Line Items]
Fair value of investments	12,300		12,800
The Coca-Cola Company [Member]
Summary of Investment Holdings [Line Items]
Fair value of investments	16,900		16,500
Investment Concentration [Member] | Equity Securities [Member]
Summary of Investment Holdings [Line Items]
Concentration percentage	59.00%		55.00%
Number of companies in concentration percentage	4		4

[1] 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-$12.3 billion; and The Coca-Cola Company-$16.9 billion).

[2] 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).