Fair Value Measurement of Assets and Liabilities - Quantitative Information About Level 3 Significant Inputs (Details) - Significant Unobservable Inputs (Level 3) - Recurring $ in Millions	Sep. 30, 2021 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Probability of vesting | Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Warrant, measurement input	0.000
Probability of vesting | Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Warrant, measurement input	0.950
Probability of vesting | Weighted Average
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Warrant, measurement input	0.71
Equity volatility | Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Warrant, measurement input	0.245
Equity volatility | Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Warrant, measurement input	0.617
Equity volatility | Weighted Average
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Warrant, measurement input	0.42
Warrant
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Warrant	$ 1,434	$ 1,051	$ 281