Segment and Geographic Information and Significant Customers - Classification of Revenue by Major Geographic Areas (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2019		Sep. 30, 2018		Sep. 30, 2017
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues	$ 470,658	$ 449,197	$ 409,583	$ 493,654	$ 479,432	$ 449,449	$ 466,193	$ 447,224	$ 1,823,092	[1]	$ 1,842,298	[1]	$ 1,728,175	[1]
UNITED STATES
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues									1,367,752		1,374,877		1,244,900
International [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues									$ 455,340		$ 467,421		$ 483,275

[1] Segment revenues differ from reported revenues due to certain revenue adjustments related to acquisitions that would otherwise have been recognized but for the purchase accounting treatment of the business combinations. These revenues are included to allow for more complete comparisons to the financial results of historical operations and in evaluating management performance.