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, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2018		Sep. 30, 2017		Sep. 30, 2016
Revenues from External Customers and Long-Lived Assets [Line Items]
Segment Revenues	[1]									$ 2,069,382		$ 1,977,391		$ 1,979,593
Total revenues		$ 532,905	$ 502,887	$ 514,224	$ 501,645	$ 465,910	$ 486,221	$ 499,573	$ 487,658	2,051,661	[1]	1,939,362	[1]	1,948,903	[1]
UNITED STATES
Revenues from External Customers and Long-Lived Assets [Line Items]
Segment Revenues										1,470,669		1,352,039		1,385,265
International [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Segment Revenues										$ 580,992		$ 587,323		$ 563,638

[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.