Accumulated Other Comprehensive Income (Loss) - Reclassification out of Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	$ 1,884,305	$ 1,781,806	$ 1,323,827
Other comprehensive income (loss) before reclassifications	(3,346)	(2,687)	3,706
Reclassifications out of other comprehensive loss			388
Balance	1,862,353	1,884,305	1,781,806
Foreign Currency Translation
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	(17,250)	(14,563)	(18,269)
Other comprehensive income (loss) before reclassifications	(1,734)	(2,687)	3,706
Balance	(18,984)	(17,250)	(14,563)
Derivative Financial Instruments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance			(388)
Other comprehensive income (loss) before reclassifications	(1,612)
Reclassifications out of other comprehensive loss			388
Balance	(1,612)
Accumulated Other Comprehensive Income (Loss) [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	(17,250)	(14,563)	(18,657)
Balance	$ (20,596)	$ (17,250)	$ (14,563)