Investments in equity securities (Parenthetical) (Detail) $ in Millions	Mar. 31, 2017 USD ($) Company		Dec. 31, 2016 USD ($) Company
Summary of Investment Holdings [Line Items]
Fair value of investments	$ 135,020	[1]	$ 122,032	[2]
Equity Securities [Member] | Investment Concentration [Member]
Summary of Investment Holdings [Line Items]
Concentration percentage	65.00%		62.00%
Number of companies in concentration percentage | Company	5		5
American Express Company [Member]
Summary of Investment Holdings [Line Items]
Fair value of investments	$ 12,000		$ 11,200
Apple Inc. [Member]
Summary of Investment Holdings [Line Items]
Fair value of investments	19,200		7,100
The Coca-Cola Company [Member]
Summary of Investment Holdings [Line Items]
Fair value of investments	17,000		16,600
IBM [Member]
Summary of Investment Holdings [Line Items]
Fair value of investments	11,200		13,500
Wells Fargo & Company [Member]
Summary of Investment Holdings [Line Items]
Fair value of investments	$ 27,800		$ 27,600

[1] Approximately 65% of the aggregate fair value was concentrated in the equity securities of five companies: American Express Company - $12.0 billion, Apple Inc. - $19.2 billion, The Coca-Cola Company - $17.0 billion, International Business Machines Corporation ("IBM") - $11.2 billion and Wells Fargo & Company - $27.8 billion.

[2] Approximately 62% of the aggregate fair value was concentrated in the equity securities of five companies: American Express Company - $11.2 billion, Apple Inc. - $7.1 billion, The Coca-Cola Company - $16.6 billion, IBM - $13.5 billion and Wells Fargo & Company - $27.6 billion.