Other investments - Narrative (Detail) - Available-for-sale Securities [Member] $ / shares in Units, $ in Millions		9 Months Ended
	Dec. 12, 2014 USD ($)	Sep. 30, 2015 USD ($) $ / shares shares
Wm. Wrigley Jr. Company [Member]
Other Investments [Line Items]
Investment in preferred stock, liquidation value | $		$ 2,100
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
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
Exercise price for warrants, per share		$ 7.142857
Class A 9% Cumulative Compounding Perpetual Preferred Stock [Member] | RBI - Parent Company of Burger King and Tim Hortons [Member]
Other Investments [Line Items]
Investment in preferred stock, stated dividend rate		9.00%
Stated value of equity securities acquired | $	$ 3,000
Class A 9% Cumulative Compounding Perpetual Preferred Stock and Common Stock [Member] | RBI - Parent Company of Burger King and Tim Hortons [Member]
Other Investments [Line Items]
Amount paid for an investment | $	$ 3,000
Percentage of voting interests	14.40%