Short-term Investment Securities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Short-term investment securities	$ 100,937	$ 100,155
Unrealized loss on investment securities, net of deferred taxes	82	23	$ 0
Deferred taxes on unrealized loss on investment securities	41	13	$ 0
Accumulated other comprehensive loss	$ (1,464)	$ (1,345)
Available-for-sale Securities
Schedule of Available-for-sale Securities [Line Items]
Weighted-average fixed rate	1.40%	1.10%
Accumulated other comprehensive loss	$ (105)	$ (23)