Investments in equity securities (Parenthetical) (Detail) $ in Millions	Sep. 30, 2017 USD ($) Company		Dec. 31, 2016 USD ($) Company
Summary of Investment Holdings [Line Items]
Fair value of investments	$ 157,650	[1]	$ 122,032	[2]
Equity Securities [Member] | Investment Concentration [Member]
Summary of Investment Holdings [Line Items]
Concentration percentage	62.00%		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	$ 13,700		$ 11,200
Apple Inc. [Member]
Summary of Investment Holdings [Line Items]
Fair value of investments	21,300		7,100
The Coca-Cola Company [Member]
Summary of Investment Holdings [Line Items]
Fair value of investments	18,000		16,600
IBM [Member]
Summary of Investment Holdings [Line Items]
Fair value of investments			13,500
Wells Fargo & Company [Member]
Summary of Investment Holdings [Line Items]
Fair value of investments	26,900		$ 27,600
Bank of America Corporation [Member]
Summary of Investment Holdings [Line Items]
Fair value of investments	$ 17,700

[1] Approximately 62% of the aggregate fair value was concentrated in the equity securities of five companies: American Express Company - $13.7 billion, Apple Inc. - $21.3 billion, Bank of America Corporation - $17.7 billion, The Coca-Cola Company - $18.0 billion and Wells Fargo & Company - $26.9 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, International Business Machines Corporation - $13.5 billion and Wells Fargo & Company - $27.6 billion.