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Investments in equity securities - Narrative (Detail) - USD ($)
12 Months Ended
Aug. 24, 2017
Dec. 12, 2014
Dec. 31, 2017
Dec. 31, 2011
Dec. 31, 2016
Equity Securities [Member]          
Summary of Investment Holdings [Line Items]          
Unrealized losses related to securities that have been in an unrealized loss position for more than 12 months     $ 94,000,000   $ 551,000,000
Bank of America Corporation [Member] | Non-Cumulative Perpetual Preferred Stock [Member] | Available-for-sale Securities [Member]          
Summary of Investment Holdings [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] | Available-for-sale Securities [Member]          
Summary of Investment Holdings [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        
RBI [Member] | Class A 9% Cumulative Compounding Perpetual Preferred Stock [Member] | Available-for-sale Securities [Member]          
Summary of Investment Holdings [Line Items]          
Investment in preferred stock, stated dividend rate   9.00%      
Stated value of equity securities   $ 3,000,000,000      
Preferred stock redemption date     Dec. 12, 2017