Goodwill and Intangible Assets - Summary of Intangible Assets (Detail) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Dec. 31, 2017
Finite Lived Intangible Assets [Line Items]
Amortizable intangible assets	$ 1,552,132	$ 1,551,949	$ 624,368
Accumulated amortization	(462,940)	(410,633)	(286,561)
Total amortizable intangible assets, net	1,089,192	1,141,316	337,807
Indefinite lived trademarks	193,050	193,050	73,050
Total intangibles, net	$ 1,282,242	1,334,366	410,857
Weighted Average [Member]
Finite Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life	18 years 2 months 12 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 5 months 19 days
Customer Relationships [Member]
Finite Lived Intangible Assets [Line Items]
Amortizable intangible assets	$ 1,530,748	1,530,565	609,984
Customer Relationships [Member] | Weighted Average [Member]
Finite Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life	18 years 4 months 17 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 4 months 28 days
Beneficial Lease Arrangements [Member]
Finite Lived Intangible Assets [Line Items]
Amortizable intangible assets	$ 8,060	$ 8,060	$ 1,060
Beneficial Lease Arrangements [Member] | Weighted Average [Member]
Finite Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life	4 years 2 months 26 days