Debt - 3.625% Senior Secured Notes Offering - Additional Information (Detail)				6 Months Ended	12 Months Ended
	Nov. 05, 2018 USD ($)	Oct. 12, 2018 EUR (€)	Oct. 09, 2018 EUR (€)	Mar. 31, 2019 USD ($)	Sep. 30, 2018 USD ($)	Mar. 31, 2019 EUR (€)	Oct. 09, 2018 USD ($)	Oct. 09, 2018 EUR (€)	Sep. 30, 2018 EUR (€)	Mar. 31, 2018
Debt Instrument [Line Items]
Aggregate principal amount of debt instrument				$ 3,022,000,000	$ 2,851,000,000
3.625% Senior Secured Notes due 2026
Debt Instrument [Line Items]
Interest rate				3.625%		3.625%
3.625% Senior Secured Notes due 2026 | Acquisition Corp.
Debt Instrument [Line Items]
Aggregate principal amount of debt instrument | €								€ 250,000,000
Due date of senior secured notes			2026	2026
Interest rate		3.625%		3.625%		3.625%	3.625%	3.625%
4.125% Senior Secured Notes due 2024 | Acquisition Corp.
Debt Instrument [Line Items]
Aggregate principal amount of debt instrument				$ 349,000,000	$ 402,000,000	€ 311,000,000			€ 345,000,000
Due date of senior secured notes		2024		2024	2024
Interest rate		4.125%		4.125%	4.125%	4.125%	4.125%	4.125%	4.125%
Redemptions of senior notes | €		€ 34,500,000	€ 34,500,000
4.875% Senior Secured Notes due 2024
Debt Instrument [Line Items]
Interest rate				4.875%		4.875%				4.875%
4.875% Senior Secured Notes due 2024 | Acquisition Corp.
Debt Instrument [Line Items]
Aggregate principal amount of debt instrument				$ 220,000,000	$ 250,000,000		$ 30,000,000
Due date of senior secured notes			2024	2024	2024
Interest rate				4.875%	4.875%	4.875%	4.875%	4.875%	4.875%
5.625% Senior Secured Notes
Debt Instrument [Line Items]
Interest rate				5.625%		5.625%				5.625%
5.625% Senior Secured Notes | Acquisition Corp.
Debt Instrument [Line Items]
Aggregate principal amount of debt instrument				$ 221,000,000	$ 248,000,000
Due date of senior secured notes	2022			2022	2022
Interest rate	5.625%			5.625%	5.625%	5.625%			5.625%
Redemptions of senior notes	$ 26,550,000