Fair Value Measurements (Details 3) - USD ($) $ in Thousands		3 Months Ended	12 Months Ended
		Sep. 30, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Warrants Not Settleable in Cash, Fair Value Disclosure				$ 5,161
Level 3
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Gain on Sale of Investments			$ 7,478	2,532	$ 1,402
Proceeds from Issuance of Warrants	[1]		(8,589)	(3,093)	(1,894)
Stock and Warrants Issued During Period, Value, Preferred Stock and Warrants			$ 821	1,407	1,911
Equity Warrants Transfers to Available For Sale Securities		$ (1,182)		(1,182)	836
Warrants Not Settleable in Cash, Fair Value Disclosure				$ 5,161	$ 5,497	$ 4,914

[1] Upon exercise, warrants become equity securities in public companies. These are often subject to lock-up restrictions that must be met before the equity security can be sold, during which time they are reported as equity investments with readily determinable fair values.