Fair Value Measurements - Assets Measured at Fair Value on Recurring Basis (Details) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities - available for sale	$ 27,597	$ 25,917
Assets, fair value disclosure	27,597	25,917
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities - available for sale	0	0
Assets, fair value disclosure	0	0
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities - available for sale	27,597	25,917
Assets, fair value disclosure	27,597	25,917
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities - available for sale	0	0
Assets, fair value disclosure	$ 0	$ 0