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, 2019	Mar. 31, 2018	Mar. 31, 2019	Mar. 31, 2018
Revenues from External Customers and Long-Lived Assets [Line Items]
Document Period End Date			Mar. 31, 2019
Segment Revenues	$ 409,583	$ 466,193	$ 903,237	$ 913,417
UNITED STATES
Revenues from External Customers and Long-Lived Assets [Line Items]
Segment Revenues	299,101	342,003	682,533	680,486
International [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Segment Revenues	$ 110,482	$ 124,190	$ 220,704	$ 232,931