Fair value measurements - Other information (Detail) - Significant Unobservable Inputs (Level 3) [Member] - Equity Index Put Options [Member] - Fair Value, Measurements, Recurring [Member] $ in Millions	Jun. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative contract liabilities, fair value	$ 2,006	$ 2,172
Option Pricing Model [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative contract liabilities, fair value	$ 2,006
Option Pricing Model [Member] | Volatility [Member] | Weighted Average [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Unobservable inputs	0.17