Fair value measurements - Other information (Detail) - Significant Unobservable Inputs (Level 3) [Member] - Equity Index Put Options [Member] - Fair Value, Measurements, Recurring [Member] $ in Millions |
Dec. 31, 2018
USD ($)
|
Dec. 31, 2017
USD ($)
|
|---|---|---|
| Fair Value Measurement Inputs and Valuation Techniques [Line Items] | ||
| Derivative contract liabilities, fair value | $ 2,452 | $ 2,172 |
| Option Pricing Model [Member] | ||
| Fair Value Measurement Inputs and Valuation Techniques [Line Items] | ||
| Derivative contract liabilities, fair value | $ 2,452 | |
| Option Pricing Model [Member] | Volatility [Member] | Weighted Average [Member] | ||
| Fair Value Measurement Inputs and Valuation Techniques [Line Items] | ||
| Unobservable inputs | 0.18 |