Investments Investments in Private Companies (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Schedule of Equity Method Investments [Line Items]
Net change in unrealized gain (loss) on private company investments	$ 19	$ 0	$ (136)	$ (1,265)
Investments in private companies, at adjusted cost	4,615		4,615		$ 4,870
Fair Value, Measurements, Nonrecurring [Member]
Schedule of Equity Method Investments [Line Items]
Net change in unrealized gain (loss) on private company investments	19	135
Equity Method Investment, Other than Temporary Impairment	$ 0	$ 0	$ 0	$ 1,500