Short-term Investment Securities (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Short-term investment securities	$ 101,714		$ 101,714		$ 100,937	$ 100,155
Unrealized gain (loss), net of deferred taxes	101	$ (11)	78	$ (24)
Tax effect of the unrealized gain (loss) on short-term investment securities	33	$ (6)	26	$ (14)
Accumulated other comprehensive loss	1,266		$ 1,266		1,464	$ 1,345
Available-for-sale Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Weighted-average fixed rate			1.40%
Accumulated other comprehensive loss	$ 27		$ 27		$ 105