Segment and Geographic Information and Significant Customers (Classification of Revenue by Major Geographic Areas) (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2020	Mar. 31, 2019
Revenues from External Customers and Long-Lived Assets [Line Items]
Segment Revenues	$ 369,337	$ 336,584	$ 787,570	$ 756,259
UNITED STATES
Revenues from External Customers and Long-Lived Assets [Line Items]
Segment Revenues	291,291	269,990	638,101	613,968
International [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Segment Revenues	$ 78,046	$ 66,594	$ 149,469	$ 142,291