Marketable Securities - Schedule of Amortized Cost and Estimated Fair Value of Marketable Securities Classified by Maturity Date (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Available For Sale Securities Debt Maturities Fair Value [Abstract]
Amortized Cost, Due within one year	$ 112,427	$ 104,742
Amortized Cost, Due after one through five years	14,540	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	126,967	109,750
Fair Value, Due within one year	112,419	104,605
Fair Value, Due after one through five years	14,547	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	$ 126,966	$ 109,589