Other investment - Narrative (Detail) - Available-for-sale Securities [Member] - USD ($)	1 Months Ended		12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2016	Dec. 31, 2009	Dec. 31, 2008
Wm. Wrigley Jr. Company [Member]
Other Investments [Line Items]
Percentage of original preferred stock investment redeemable subject to put and call arrangements	50.00%		50.00%
Preferred stock redemption period			90 days
Preferred stock redemption period start date			Oct. 06, 2016
Wm. Wrigley Jr. Company [Member] | Preferred Stock [Member]
Other Investments [Line Items]
Purchase of investment					$ 2,100,000,000
Sale of investment		$ 4,560,000,000
The Dow Chemical Company [Member] | Conversion of Series A Cumulative Convertible Perpetual Preferred Stock into Common Stock [Member]
Other Investments [Line Items]
Common shares issued upon conversion of convertible preferred stock	72,600,000
The Dow Chemical Company [Member] | Series A Cumulative Convertible Perpetual Preferred Stock [Member]
Other Investments [Line Items]
Purchase of investment				$ 3,000,000,000
Number of shares acquired				3,000,000
Investment in preferred stock, stated dividend rate			8.50%
Bank of America Corporation [Member] | Non-Cumulative Perpetual Preferred Stock [Member]
Other Investments [Line Items]
Preferred stock redemption period start date			May 07, 2019
Investment in preferred stock, stated dividend rate			6.00%
Investment, number of shares owned	50,000		50,000
Investment in preferred stock, liquidation value per share	$ 100,000		$ 100,000
Redemption value of Perpetual Preferred Stock, per share	$ 105,000		$ 105,000
Redemption value of Perpetual Preferred Stock, aggregate value	$ 5,250,000,000		$ 5,250,000,000
Bank of America Corporation [Member] | Warrants Expiring 2021 [Member]
Other Investments [Line Items]
Number of common shares that can be purchased	700,000,000		700,000,000
Aggregate cost to exercise warrants	$ 5,000,000,000		$ 5,000,000,000
Exercise price for warrants, per share			$ 7.142857
RBI [Member] | Class A 9% Cumulative Compounding Perpetual Preferred Stock [Member]
Other Investments [Line Items]
Preferred stock redemption period start date			Dec. 12, 2017
Investment in preferred stock, stated dividend rate			9.00%
Stated value of equity securities	$ 3,000,000,000		$ 3,000,000,000
Preferred stock redemption at option of holder start date			Dec. 12, 2024
Preferred stock redemption price percentage			109.90%