Goodwill and Intangible Assets - Summary of Intangible Assets (Detail) - USD ($) $ in Thousands	9 Months Ended
	Jun. 30, 2019	Sep. 30, 2018	Jun. 30, 2018
Finite Lived Intangible Assets [Line Items]
Amortizable intangible assets	$ 1,552,445	$ 1,551,949	$ 1,551,402
Accumulated amortization	(567,801)	(410,633)	(373,447)
Total amortizable intangible assets, net	984,644	1,141,316	1,177,955
Indefinite lived trademarks	193,050	193,050	193,050
Total intangibles, net	$ 1,177,694	1,334,366	1,371,005
Weighted Average [Member]
Finite Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life	17 years 9 months 18 days
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 13 days
Customer Relationships [Member]
Finite Lived Intangible Assets [Line Items]
Amortizable intangible assets	$ 1,531,061	1,530,565	1,530,434
Customer Relationships [Member] | Weighted Average [Member]
Finite Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life	17 years 10 months 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 29 days
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	3 years 9 months 18 days