Fair value measurements - Other information (Detail) $ in Millions	9 Months Ended
	Sep. 30, 2021 USD ($)		Dec. 31, 2020 USD ($)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value of investments	$ 310,739	[1]	$ 281,170	[2]
Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value of investments	12,098		$ 9,284
Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member] | Option Pricing Model [Member] | Equity Index Put Option Contract Liabilities [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative contract liabilities, fair value	286
Significant Unobservable Inputs (Level 3) [Member] | Preferred Stock [Member] | Fair Value, Measurements, Recurring [Member] | Discounted Cash Flow [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value of investments	11,179
Significant Unobservable Inputs (Level 3) [Member] | Common Stock Warrants [Member] | Fair Value, Measurements, Recurring [Member] | Warrant Pricing Model [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value of investments	$ 612
Significant Unobservable Inputs (Level 3) [Member] | Expected Duration [Member] | Weighted Average [Member] | Preferred Stock [Member] | Fair Value, Measurements, Recurring [Member] | Discounted Cash Flow [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Unobservable inputs, term	7 years
Significant Unobservable Inputs (Level 3) [Member] | Expected Duration [Member] | Weighted Average [Member] | Common Stock Warrants [Member] | Fair Value, Measurements, Recurring [Member] | Warrant Pricing Model [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Unobservable inputs, term	8 years
Significant Unobservable Inputs (Level 3) [Member] | Discount for Transferability Restrictions and Subordination [Member] | Weighted Average [Member] | Preferred Stock [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Unobservable inputs	0.0355
Significant Unobservable Inputs (Level 3) [Member] | Volatility [Member] | Weighted Average [Member] | Fair Value, Measurements, Recurring [Member] | Option Pricing Model [Member] | Equity Index Put Option Contract Liabilities [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Unobservable inputs	0.18
Significant Unobservable Inputs (Level 3) [Member] | Volatility [Member] | Weighted Average [Member] | Common Stock Warrants [Member] | Fair Value, Measurements, Recurring [Member] | Warrant Pricing Model [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Unobservable inputs	0.37

[1] Approximately 70% of the aggregate fair value was concentrated in four companies (American Express Company – $25.4 billion; Apple Inc. – $128.4 billion; Bank of America Corporation – $43.9 billion and The Coca-Cola Company – $21.0 billion).

[2] Approximately 68% of the aggregate fair value was concentrated in four companies (American Express Company – $18.3 billion; Apple Inc. – $120.4 billion; Bank of America Corporation – $31.3 billion and The Coca-Cola Company – $21.9 billion).