Fair Value Measures (Assets and Liabilities Measured at Fair Value on Recurring Basis) (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Jun. 30, 2019	Sep. 30, 2018	Jun. 30, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale Securities, Weighted Average Maturity	6 months	7 months 9 days
Document Period End Date			Jun. 30, 2019
Fair Value, Measurements, Recurring
Assets:
Money market funds	$ 386,071,000	$ 200,004,000	$ 386,071,000
Bank Time Deposits, Fair Value Disclosure	91,147,000	88,158,000	91,147,000
Commercial Paper, Fair value	39,409,000	27,363,000	39,409,000
Corporate notes and bonds, Fair Value	42,358,000	57,417,000	42,358,000
Foreign Currency Contract, Asset, Fair Value Disclosure	100,000	143,000	100,000
Total assets at fair value	559,085,000	373,085,000	559,085,000
Liabilities:
Contingent earn-out	(889,000)	(4,000,000)	(889,000)
Foreign Currency Contracts, Liability, Fair Value Disclosure	(109,000)	(1,192,000)	(109,000)
Total liabilities at fair value	(998,000)	(5,192,000)	(998,000)
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Assets:
Money market funds	386,071,000	200,004,000	386,071,000
Bank Time Deposits, Fair Value Disclosure	0	0	0
Commercial Paper, Fair value	0	0	0
Corporate notes and bonds, Fair Value	0	0	0
Foreign Currency Contract, Asset, Fair Value Disclosure	0	0	0
Total assets at fair value	386,071,000	200,004,000	386,071,000
Liabilities:
Contingent earn-out	0	0	0
Foreign Currency Contracts, Liability, Fair Value Disclosure	0	0	0
Total liabilities at fair value	0	0	0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Assets:
Money market funds	0	0	0
Bank Time Deposits, Fair Value Disclosure	91,147,000	88,158,000	91,147,000
Commercial Paper, Fair value	39,409,000	27,363,000	39,409,000
Corporate notes and bonds, Fair Value	42,358,000	57,417,000	42,358,000
Foreign Currency Contract, Asset, Fair Value Disclosure	100,000	143,000	100,000
Total assets at fair value	173,014,000	173,081,000	173,014,000
Liabilities:
Contingent earn-out	0	0	0
Foreign Currency Contracts, Liability, Fair Value Disclosure	(109,000)	(1,192,000)	(109,000)
Total liabilities at fair value	(109,000)	(1,192,000)	(109,000)
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Assets:
Money market funds	0	0	0
Bank Time Deposits, Fair Value Disclosure	0	0	0
Commercial Paper, Fair value	0	0	0
Corporate notes and bonds, Fair Value	0	0	0
Foreign Currency Contract, Asset, Fair Value Disclosure	0	0	0
Total assets at fair value	0	0	0
Liabilities:
Contingent earn-out	(889,000)	(4,000,000)	(889,000)
Foreign Currency Contracts, Liability, Fair Value Disclosure	0	0	0
Total liabilities at fair value	(889,000)	(4,000,000)	(889,000)
Corporate Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Amortized Cost Basis	42,291,674	57,563,000	42,291,674
Commercial Paper [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Amortized Cost Basis	$ 39,113,394	$ 27,194,000	$ 39,113,394