Marketable Securities - Maturity Schedule (Details) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Marketable Securities, Interest Income	$ 0	$ 110
Amortized Cost
Amortized Cost	25,712,000	22,618,000
Fair Value
Fair Value	25,712,000	22,619,000
Debt Securities
Amortized Cost
Due in less than one year	3,864,000	5,515,000
Due after one year and within two years	0	3,859,000
Amortized Cost	3,864,000	9,374,000
Fair Value
Due in less than one year	3,864,000	5,516,000
Due after one year and within two years	0	3,859,000
Fair Value	$ 3,864,000	$ 9,375,000