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, 2017	Mar. 31, 2016	Mar. 31, 2017	Mar. 31, 2016
Revenues from External Customers and Long-Lived Assets [Line Items]
Segment Revenues	$ 511,097	$ 487,413	$ 1,007,115	$ 982,283
Total revenues	499,573	478,733	987,231	964,848
UNITED STATES
Revenues from External Customers and Long-Lived Assets [Line Items]
Segment Revenues	352,937	338,710	702,107	694,524
International [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Segment Revenues	$ 146,636	$ 140,023	$ 285,124	$ 270,324