Fair Value of Financial Instruments - Quantitative Information About Significant Unobservable Inputs (Details) $ in Thousands	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Sales restrictions discount | Equity warrant assets (public portfolio) | Lower Limit
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Weighted Average	0.10
Sales restrictions discount | Equity warrant assets (public portfolio) | Upper Limit
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Weighted Average	0.20
Significant Unobservable Inputs (Level 3) | Equity warrant assets (private portfolio)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 191,181	$ 160,692
Significant Unobservable Inputs (Level 3) | Equity warrant assets (public portfolio)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 1,036	346
Significant Unobservable Inputs (Level 3) | Venture capital and private equity fund investments
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value		$ 134
Significant Unobservable Inputs (Level 3) | Volatility | Equity warrant assets (private portfolio) | Lower Limit
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Weighted Average	0.244	0.236
Significant Unobservable Inputs (Level 3) | Volatility | Equity warrant assets (private portfolio) | Upper Limit
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Weighted Average	0.568	0.548
Significant Unobservable Inputs (Level 3) | Volatility | Equity warrant assets (private portfolio) | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Weighted Average	0.432	0.382
Significant Unobservable Inputs (Level 3) | Volatility | Equity warrant assets (public portfolio) | Lower Limit
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Weighted Average	0.460	0.392
Significant Unobservable Inputs (Level 3) | Volatility | Equity warrant assets (public portfolio) | Upper Limit
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Weighted Average	0.568	0.548
Significant Unobservable Inputs (Level 3) | Volatility | Equity warrant assets (public portfolio) | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Weighted Average	0.491	0.507
Significant Unobservable Inputs (Level 3) | Risk-Free Interest Rate | Equity warrant assets (private portfolio) | Lower Limit
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Weighted Average	0.0001	0.005
Significant Unobservable Inputs (Level 3) | Risk-Free Interest Rate | Equity warrant assets (private portfolio) | Upper Limit
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Weighted Average	0.005	0.019
Significant Unobservable Inputs (Level 3) | Risk-Free Interest Rate | Equity warrant assets (private portfolio) | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Weighted Average	0.001	0.016
Significant Unobservable Inputs (Level 3) | Risk-Free Interest Rate | Equity warrant assets (public portfolio)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Weighted Average		1.9
Significant Unobservable Inputs (Level 3) | Risk-Free Interest Rate | Equity warrant assets (public portfolio) | Lower Limit
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Weighted Average	0.003
Significant Unobservable Inputs (Level 3) | Risk-Free Interest Rate | Equity warrant assets (public portfolio) | Upper Limit
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Weighted Average	0.009
Significant Unobservable Inputs (Level 3) | Risk-Free Interest Rate | Equity warrant assets (public portfolio) | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Weighted Average	0.006	0.019
Significant Unobservable Inputs (Level 3) | Sales restrictions discount | Equity warrant assets (public portfolio) | Lower Limit
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Weighted Average	0.100	0.100
Significant Unobservable Inputs (Level 3) | Sales restrictions discount | Equity warrant assets (public portfolio) | Upper Limit
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Weighted Average	0.200	0.200
Significant Unobservable Inputs (Level 3) | Sales restrictions discount | Equity warrant assets (public portfolio) | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Weighted Average	0.102	0.136
Significant Unobservable Inputs (Level 3) | Marketability discount | Equity warrant assets (private portfolio)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Weighted Average	20.6	17.5
Significant Unobservable Inputs (Level 3) | Marketability discount | Equity warrant assets (private portfolio) | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Weighted Average	0.206	0.175
Significant Unobservable Inputs (Level 3) | Remaining life assumption | Equity warrant assets (private portfolio)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Weighted Average	40.0	45.0
Significant Unobservable Inputs (Level 3) | Remaining life assumption | Equity warrant assets (private portfolio) | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Weighted Average	0.400	0.450