Fair Value of Financial Instruments - Quantitative Information About Significant Unobservable Inputs (Additional Information) (Detail)	6 Months Ended
	Jun. 30, 2018
Equity warrant assets, private portfolio
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Weighted average contractual remaining term	6 years 22 days
Estimated remaining life	2 years 8 months 23 days
Lower Limit | Level 3 | Equity warrant asset, public portfolio
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Sales restrictions discount rate	10.00%
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]
Sales restrictions discount rate	20.00%
Fair value inputs, sales restriction, period	6 months