Fair Value of Financial Instruments - Quantitative Information About Significant Unobservable Inputs (Additional Information) (Detail)	9 Months Ended
	Sep. 30, 2018
Equity warrant assets, private portfolio
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Weighted average contractual remaining term	6 years 2 months 12 days
Estimated remaining life	2 years 9 months 18 days
Lower Limit | Level 3 | Equity warrant asset, public portfolio
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Fair value inputs, sales restriction, period	3 months
Upper Limit | Level 3 | Equity warrant asset, public portfolio
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Fair value inputs, sales restriction, period	6 months
Sales restrictions discount | Lower Limit | Level 3 | Equity warrant asset, public portfolio
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Weighted Average	0.10
Sales restrictions discount | Upper Limit | Level 3 | Equity warrant asset, public portfolio
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Weighted Average	0.20