Marketable Securities - Schedule of Amortized Cost and Estimated Fair Value of Marketable Securities Classified by Maturity Date (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Available For Sale Securities Debt Maturities Fair Value [Abstract]
Amortized Cost, Due within one year	$ 104,742	$ 82,499
Amortized Cost, Due after one through five years	5,008	5,431
Amortized Cost, Due after five through ten years	0	0
Amortized Cost, Due after ten years	0	0
Amortized Cost, Total marketable securities	109,750	87,930
Fair Value, Due within one year	104,605	82,445
Fair Value, Due after one through five years	4,984	5,427
Fair Value, Due after five through ten years	0	0
Fair Value, Due after ten years	0	0
Fair Value, Total marketable securities	$ 109,589	$ 87,872