Marketable Securities - Schedule of Amortized Cost and Estimated Fair Value of Marketable Securities Classified by Maturity Date (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Available For Sale Securities Debt Maturities Fair Value [Abstract]
Amortized Cost, Due within one year	$ 99,714	$ 104,742
Amortized Cost, Due after one through five years	12,689	5,008
Amortized Cost, Due after five through ten years	0	0
Amortized Cost, Due after ten years	0	0
Amortized Cost, Total marketable securities	112,403	109,750
Fair Value, Due within one year	99,609	104,605
Fair Value, Due after one through five years	12,657	4,984
Fair Value, Due after five through ten years	0	0
Fair Value, Due after ten years	0	0
Fair Value, Total marketable securities	$ 112,266	$ 109,589