Short-term Investment Securities (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Short-term investment securities	$ 101,254		$ 100,937	$ 100,155
Unrealized gain (loss), net of deferred taxes	(23)	$ (13)
Tax effect of the unrealized gain (loss) on short-term investment securities	(8)	$ (8)
Accumulated other comprehensive loss	$ 1,429		1,464	$ 1,345
Available-for-sale Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Weighted-average fixed rate	1.50%
Accumulated other comprehensive loss	$ 128		$ 105