Fair Value Measurements - Narrative (Detail) - USD ($)	Sep. 30, 2018	Mar. 31, 2018
Debt Instrument [Line Items]
Unrealized loss position of securities for more than 12 months	$ 0	$ 0
Convertible Debt \| Level 2 \| Convertible Senior Notes Due 2023
Debt Instrument [Line Items]
Fair value of Notes	$ 488,800,000

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for more than 12 months.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563

Name:	us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLonger
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant