Debt - Notes Offering - Additional Information (Detail) - USD ($)		6 Months Ended	12 Months Ended
Mar. 14, 2018	Mar. 31, 2019	Sep. 30, 2018
Debt Instrument [Line Items]
Face or principal amount of debt instrument		$ 3,022,000,000	$ 2,851,000,000
5.500% Senior Notes due 2026 \| Acquisition Corp.
Debt Instrument [Line Items]
Face or principal amount of debt instrument	$ 325,000,000	$ 325,000,000	$ 325,000,000
Due date of senior secured notes	2026	2026	2026
Interest rate	5.50%	5.50%	5.50%

Face (par) amount of debt instrument at time of issuance.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=114775744&loc=d3e28551-108399

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=114775985&loc=d3e28878-108400

Name:	us-gaap_DebtInstrumentFaceAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant