Goodwill and Intangible Assets - Summary of Intangible Assets (Detail) - USD ($) $ in Thousands	6 Months Ended
	Mar. 31, 2019	Sep. 30, 2018	Mar. 31, 2018
Finite Lived Intangible Assets [Line Items]
Amortizable intangible assets	$ 1,552,286	$ 1,551,949	$ 1,540,734
Accumulated amortization	(515,387)	(410,633)	(323,482)
Total amortizable intangible assets, net	1,036,899	1,141,316	1,217,252
Indefinite lived trademarks	193,050	193,050	193,050
Total intangibles, net	$ 1,229,949	1,334,366	1,410,302
Weighted Average [Member]
Finite Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life	18 years
Minimum [Member]
Finite Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life	5 years
Maximum [Member]
Finite Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life	20 years
Noncompete Agreements [Member]
Finite Lived Intangible Assets [Line Items]
Amortizable intangible assets	$ 2,824	2,824	2,824
Noncompete Agreements [Member] | Weighted Average [Member]
Finite Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life	2 years 4 months 17 days
Customer Relationships [Member]
Finite Lived Intangible Assets [Line Items]
Amortizable intangible assets	$ 1,530,902	1,530,565	1,519,766
Customer Relationships [Member] | Weighted Average [Member]
Finite Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life	18 years 1 month 20 days
Trademarks [Member]
Finite Lived Intangible Assets [Line Items]
Amortizable intangible assets	$ 10,500	10,500	10,500
Trademarks [Member] | Weighted Average [Member]
Finite Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life	7 years 3 months
Beneficial Lease Arrangements [Member]
Finite Lived Intangible Assets [Line Items]
Amortizable intangible assets	$ 8,060	$ 8,060	$ 7,644
Beneficial Lease Arrangements [Member] | Weighted Average [Member]
Finite Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life	4 years 7 days