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Other investments - Narrative (Detail) - Available-for-sale Securities [Member] - USD ($)
9 Months Ended 12 Months Ended
Aug. 24, 2017
Sep. 30, 2017
Dec. 31, 2011
RBI [Member] | Class A 9% Cumulative Compounding Perpetual Preferred Stock [Member]      
Other Investments [Line Items]      
Investment in preferred stock, stated dividend rate   9.00%  
Stated value of equity securities   $ 3,000,000,000  
Preferred stock redemption period start date   Dec. 12, 2017  
Preferred stock redemption price percentage   109.90%  
Bank of America Corporation [Member] | Non-Cumulative Perpetual Preferred Stock [Member]      
Other Investments [Line Items]      
Number of shares acquired     50,000
Investment in preferred stock, stated dividend rate     6.00%
Investment in preferred stock, liquidation value per share     $ 100,000
Bank of America Corporation [Member] | Common Stock [Member]      
Other Investments [Line Items]      
Number of common shares that can be purchased     700,000,000
Exercise price for warrants, per share     $ 7.142857
Aggregate cost to exercise warrants $ 5,000,000,000   $ 5,000,000,000
Number of shares acquired upon exercise of warrants 700,000,000