Fair Value of Financial Instruments - Quantitative Information About Significant Unobservable Inputs (Details) - Significant Unobservable Inputs (Level 3) $ in Thousands	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Equity warrant assets (private portfolio)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 142,442	$ 119,395
Equity warrant assets (public portfolio)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	2,757	1,936
Venture capital and private equity fund investments
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 1,079	$ 919
Volatility | Equity warrant assets (private portfolio)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Weighted Average	0.385	0.367
Volatility | Equity warrant assets (public portfolio)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Weighted Average	0.547	0.479
Risk-Free Interest Rate | Equity warrant assets (private portfolio)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Weighted Average	0.025	0.018
Risk-Free Interest Rate | Equity warrant assets (public portfolio)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Weighted Average	0.026	0.021
Sales restrictions discount | Equity warrant assets (public portfolio)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Weighted Average	0.185	0.155
Marketability discount | Equity warrant assets (private portfolio)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Weighted Average	0.177	0.164
Remaining life assumption | Equity warrant assets (private portfolio)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Weighted Average	0.450	0.450