Marketable Securities - Maturity Schedule (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Marketable Securities, Interest Income	$ 0	$ 0
Amortized Cost
Amortized Cost	8,320	25,712
Fair Value
Fair Value	8,319	25,712
Debt Securities
Amortized Cost
Due in less than one year	1,002	3,864
Due after one year and within two years	0	0
Amortized Cost	1,002	3,864
Fair Value
Due in less than one year	1,001	3,864
Due after one year and within two years	0	0
Fair Value	$ 1,001	$ 3,864