Fair Value Measurements - Summary of Fair Value of Freestanding Derivative (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Mar. 31, 2019	Mar. 31, 2018
Derivatives, Fair Value [Line Items]
Fair Values of Derivative Financial Instruments Asset (Liability)		$ 81,264	$ 59,116
Not Designated as Hedging Instrument [Member] \| Dividend Receivable [Member]
Derivatives, Fair Value [Line Items]
Fair Values of Derivative Financial Instruments Asset (Liability)	$ 34,547	$ 81,264	$ 59,116

Fair value of the assets less the liabilities of a derivative or group of derivatives.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=118260190&loc=d3e13433-108611

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