Fair Value Measurements - Classification of Available-for-Sale Short-Term Investments by Contractual Maturities (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Mar. 31, 2018
Fair Value Disclosures [Abstract]
Due within one year	$ 393,734	$ 96,924
Due after one year through two years	140,577	18,517
Total	$ 534,311	$ 115,441

Amount of investments including trading securities, available-for-sale securities, held-to-maturity securities, and short-term investments classified as other and current.

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant