Fair Value Measurements - Summary of Amortized Cost and Estimated Fair Value of Available-for-Sale Marketable Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Fair Value Disclosures [Abstract]
Mature in one year or less, Amortized cost	$ 178,041
Mature after one year through two years, Amortized cost	3,585
Marketable securities, Amortized cost	181,626	$ 151,735
Mature in one year or less, Unrealized gains	149
Mature after one year through two years, Unrealized gains	8
Total available-for-sale marketable securities, Unrealized gains	157	19
Mature in one year or less, Unrealized losses	(8)
Total available-for-sale marketable securities, Unrealized losses	(8)	(191)
Mature in one year or less, Estimated Fair Value	178,182
Mature after one year through two years, Estimated Fair Value	3,593
Total available-for-sale marketable securities, Estimated Fair Value	$ 181,775	$ 151,563