Marketable Securities - Schedule of Fair Value and Amortized Cost of Company's Available-for-Sale Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Investments Debt And Equity Securities [Abstract]
Due in one year or less, Amortized Cost	$ 111,255	$ 101,027
Total, Amortized Cost	111,255	101,027
Due in one year or less, Fair Value	111,425	100,986
Total, Fair Value	$ 111,425	$ 100,986