Investments in equity securities - Narrative (Detail) - Bank of America Corporation [Member] - USD ($)		12 Months Ended
	Aug. 24, 2017	Dec. 31, 2011
Non-Cumulative Perpetual Preferred Stock [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
Common Stock [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