Fair Value of Financial Instruments - Quantitative Information About Significant Unobservable Inputs (Additional Information) (Detail)	3 Months Ended
	Mar. 31, 2018
Equity warrant assets, private portfolio
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Weighted average contractual remaining term	5 years 9 months 85 days
Estimated remaining life	2 years 7 months 30 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