Other investments - Narrative (Detail) - Available-for-sale Securities [Member]	3 Months Ended
	Mar. 31, 2016 USD ($) $ / shares shares
Wm. Wrigley Jr. Company [Member]
Other Investments [Line Items]
Investment in preferred stock, liquidation value | $	$ 2,100,000,000
Investment in preferred stock, stated dividend rate	5.00%
Percentage of original preferred stock investment subject to put and call arrangements in 2016	50.00%
Series A Cumulative Convertible Perpetual Preferred Stock [Member] | The Dow Chemical Company [Member]
Other Investments [Line Items]
Investment in preferred stock, stated dividend rate	8.50%
Investment, number of shares owned | shares	3,000,000
Investment in preferred stock, liquidation value per share	$ 1,000
Conversion ratio of preferred stock into common stock	24.201
Preferred stock conversion price, per share	$ 41.32
Common stock threshold price, per share, to trigger the issuer's option to convert preferred stock into common stock	$ 53.72
Minimum number of trading days in a consecutive-day period that the issuer's common stock price must exceed to trigger the issuer's option to convert preferred stock to common stock	20 days
Range of trading days in a consecutive day period, number of days	30 days
Non-Cumulative Perpetual Preferred Stock [Member] | Bank of America Corporation [Member]
Other Investments [Line Items]
Investment in preferred stock, stated dividend rate	6.00%
Investment, number of shares owned | shares	50,000
Investment in preferred stock, liquidation value per share	$ 100,000
Redemption value of Perpetual Preferred Stock, per share	$ 105,000
Redemption value of Perpetual Preferred Stock, aggregate value | $	$ 5,250,000,000
Warrants Expiring 2021 [Member] | Bank of America Corporation [Member]
Other Investments [Line Items]
Number of common shares that can be purchased | shares	700,000,000
Aggregate cost to exercise warrants | $	$ 5,000,000,000
Exercise price for warrants, per share	$ 7.142857
Class A 9% Cumulative Compounding Perpetual Preferred Stock [Member] | RBI [Member]
Other Investments [Line Items]
Investment in preferred stock, stated dividend rate	9.00%
Stated value of equity securities | $	$ 3,000,000,000
Common Stock [Member] | RBI [Member]
Other Investments [Line Items]
Investment, number of shares owned | shares	8,400,000