Fair Value of Financial Instruments - Quantitative Information About Significant Unobservable Inputs (Footnote Information) (Details)	12 Months Ended
	Dec. 31, 2018
Lower Limit
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Duration of the sale restrictions	3 months
Upper Limit
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Duration of the sale restrictions	6 months
Equity warrant assets (public portfolio) | Sales restrictions discount | Lower Limit
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Weighted Average	0.1
Equity warrant assets (public portfolio) | Sales restrictions discount | Upper Limit
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Weighted Average	0.2
Equity warrant assets (private portfolio)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Weighted average contractual remaining term	6 years 26 days
Estimated remaining life	2 years 8 months 23 days