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 |