Fair value measurements - Other information (Detail) $ in Millions	3 Months Ended
	Mar. 31, 2025 USD ($)	Dec. 31, 2024 USD ($)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments in equity securities	$ 263,735	$ 271,588
Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments in equity securities	9,786	$ 9,668
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]
Investments in equity securities	8,618
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]
Investments in equity securities	$ 1,162
Significant Unobservable Inputs (Level 3) [Member] | Measurement Input, Expected Term | 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	5 years
Significant Unobservable Inputs (Level 3) [Member] | Measurement Input, Expected Term | 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	6 years
Significant Unobservable Inputs (Level 3) [Member] | Discounts for Liquidity and Subordination [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	0.0325
Significant Unobservable Inputs (Level 3) [Member] | Measurement Input, Option Volatility | 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	0.42