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Other investments - Narrative (Detail) - Available-for-sale Securities [Member] - USD ($)
3 Months Ended 6 Months Ended
Sep. 30, 2017
Jun. 30, 2017
Bank of America Corporation [Member] | Non-Cumulative Perpetual Preferred Stock [Member]    
Other Investments [Line Items]    
Investment, number of shares owned   50,000
Investment in preferred stock, stated dividend rate   6.00%
Investment in preferred stock, liquidation value per share   $ 100,000
Preferred stock redemption period start date   May 07, 2019
Redemption value of Perpetual Preferred Stock, per share   $ 105,000
Redemption value of Perpetual Preferred Stock, aggregate value   $ 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
Aggregate cost to exercise warrants   $ 5,000,000,000
Exercise price for warrants, per share   $ 7.142857
Bank of America Corporation [Member] | Scenario, Forecast [Member]    
Other Investments [Line Items]    
Quarterly common stock dividend $ 0.12  
RBI [Member] | Class A 9% Cumulative Compounding Perpetual Preferred Stock [Member]    
Other Investments [Line Items]    
Investment in preferred stock, stated dividend rate   9.00%
Preferred stock redemption period start date   Dec. 12, 2017
Stated value of equity securities   $ 3,000,000,000
Preferred stock redemption at option of holder start date   Dec. 12, 2024
Preferred stock redemption price percentage   109.90%