Fair Value Measurements - Summary of Fair Value of Investments Based upon Contractual Maturities (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Fair Value Disclosures [Abstract]
Less than one year	$ 129,435	$ 134,255
Due in 1 - 5 years	93,023	105,850
Total	$ 222,458	$ 240,105