Fair Value Measurements - Classification of Available-for-Sale Short-Term Investments by Contractual Maturities (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Mar. 31, 2017
Fair Value Disclosures [Abstract]
Due within one year	$ 86,155	$ 92,874
Due in one to two years	30,588	25,227
Total	$ 116,743	$ 118,101