Fair Value of Financial Instruments - Quantitative Information About Significant Unobservable Inputs (Details) - Level 3 $ in Thousands	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Equity warrant asset, public portfolio
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Fair value	$ 6,505	$ 2,757
Equity warrant assets, private portfolio
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Fair value	$ 150,244	$ 142,442
Volatility | Equity warrant asset, public portfolio
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Weighted Average	0.542	0.547
Volatility | Equity warrant assets, private portfolio
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Weighted Average	0.386	0.385
Risk-Free interest rate | Equity warrant asset, public portfolio
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Weighted Average	0.023	0.026
Risk-Free interest rate | Equity warrant assets, private portfolio
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Weighted Average	0.022	0.025
Sales restrictions discount | Equity warrant asset, public portfolio
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Weighted Average	0.153	0.185
Marketability discount | Equity warrant assets, private portfolio
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Weighted Average	0.177	0.177
Remaining life assumption | Equity warrant assets, private portfolio
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Weighted Average	0.450	0.450
Unconsolidated venture capital and private equity fund investments
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Fair value	$ 1,035	$ 1,079