Fair Value Measurements - Summary of Amortized Cost and Estimated Fair Value of Available-for-Sale Marketable Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Fair Value Disclosures [Abstract]
Mature in one year or less, Amortized cost	$ 140,316
Mature after one year through two years, Amortized cost	11,419
Marketable securities, Amortized cost	151,735	$ 192,399
Mature in one year or less, Unrealized gains	3
Mature after one year through two years, Unrealized gains	16
Total available-for-sale marketable securities, Unrealized gains	19	1
Mature in one year or less, Unrealized losses	(190)
Mature after one year through two years, Unrealized losses	(1)
Total available-for-sale marketable securities, Unrealized losses	(191)	(297)
Mature in one year or less, Estimated Fair Value	140,129
Mature after one year through two years, Estimated Fair Value	11,434
Total available-for-sale marketable securities, Estimated Fair Value	$ 151,563	$ 192,103