Investments in equity securities (Parenthetical) (Detail) $ in Millions	Dec. 31, 2016 USD ($) Company		Dec. 31, 2015 USD ($) Company
Summary of Investment Holdings [Line Items]
Fair value of investments	$ 122,032	[1]	$ 112,137	[2]
Equity Securities [Member] | Investment Concentration [Member]
Summary of Investment Holdings [Line Items]
Concentration percentage	56.00%		59.00%
Number of companies in concentration percentage | Company	4		4
American Express Company [Member]
Summary of Investment Holdings [Line Items]
Fair value of investments	$ 11,200		$ 10,500
Wells Fargo & Company [Member]
Summary of Investment Holdings [Line Items]
Fair value of investments	27,600		27,200
IBM [Member]
Summary of Investment Holdings [Line Items]
Fair value of investments	13,500		11,200
The Coca-Cola Company [Member]
Summary of Investment Holdings [Line Items]
Fair value of investments	$ 16,600		$ 17,200

[1] Approximately 56% of the aggregate fair value was concentrated in the equity securities of four companies (American Express Company-$11.2 billion; Wells Fargo & Company-$27.6 billion; International Business Machines Corporation ("IBM")-$13.5 billion; and The Coca-Cola Company-$16.6 billion).

[2] Approximately 59% of the aggregate fair value was concentrated in the equity securities of four companies (American Express Company-$10.5 billion; Wells Fargo & Company-$27.2 billion; IBM-$11.2 billion; and The Coca-Cola Company-$17.2 billion).