Investments Investments in Private Companies (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Schedule of Equity Method Investments [Line Items]
Net change in unrealized loss on private company investments	$ (155,000)	$ (1,512,000)	$ (155,000)	$ (1,265,000)
Investments in private companies, at adjusted cost	4,620,000		4,620,000		$ 4,870,000
Fair Value, Measurements, Nonrecurring [Member]
Schedule of Equity Method Investments [Line Items]
Net change in unrealized loss on private company investments	154,000
Equity Method Investment, Other than Temporary Impairment	$ 0	$ 1,500,000	$ 0	$ 1,500,000