Fair Value of Financial Instruments - Quantitative Information About Significant Unobservable Inputs (Details) - Significant Unobservable Inputs (Level 3) $ in Thousands	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Equity warrant assets (private portfolio)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 160,692	$ 142,442
Equity warrant assets (public portfolio)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	346	2,757
Venture capital and private equity fund investments
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 134	$ 1,079
Volatility | Equity warrant assets (private portfolio)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Weighted Average	0.382	0.385
Volatility | Equity warrant assets (public portfolio)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Weighted Average	0.507	0.547
Risk-Free Interest Rate | Equity warrant assets (private portfolio)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Weighted Average	0.016	0.025
Risk-Free Interest Rate | Equity warrant assets (public portfolio)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Weighted Average	0.019	0.026
Sales restrictions discount | Equity warrant assets (public portfolio)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Weighted Average	0.136	0.185
Marketability discount | Equity warrant assets (private portfolio)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Weighted Average	0.175	0.177
Remaining life assumption | Equity warrant assets (private portfolio)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Weighted Average	0.450	0.450