Available-for-Sale-Securities - Summary of Available-for-Sale Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 37,850	$ 51,913
Gross Unrealized Gains	0	0
Gross Unrealized Losses	32	28
Estimated Fair Value	37,818	51,885
Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	33,536	42,701
Gross Unrealized Gains	0	0
Gross Unrealized Losses	32	28
Estimated Fair Value	33,504	42,673
Money Market Funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	4,314	9,212
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	$ 4,314	$ 9,212