Fair Value Measures (Assets and Liabilities Measured at Fair Value on Recurring Basis) (Detail) - USD ($)	3 Months Ended		6 Months Ended
	Mar. 31, 2019	Sep. 30, 2018	Mar. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale Securities, Weighted Average Maturity	7 months 1 day	7 months 10 days
Document Period End Date			Mar. 31, 2019
Fair Value, Measurements, Recurring
Assets:
Money market funds	$ 380,124,000	$ 200,004,000	$ 380,124,000
Bank Time Deposits, Fair Value Disclosure	97,225,000	88,158,000	97,225,000
Commercial Paper, Fair value	38,061,000	27,363,000	38,061,000
Corporate notes and bonds, Fair Value	46,686,000	57,417,000	46,686,000
Foreign Currency Contract, Asset, Fair Value Disclosure	689,000	143,000	689,000
Total assets at fair value	562,785,000	373,085,000	562,785,000
Liabilities:
Contingent earn-out	(1,450,000)	(4,000,000)	(1,450,000)
Foreign Currency Contracts, Liability, Fair Value Disclosure	(350,000)	(1,192,000)	(350,000)
Total liabilities at fair value	(1,800,000)	(5,192,000)	(1,800,000)
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Assets:
Money market funds	380,124,000	200,004,000	380,124,000
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	380,124,000	200,004,000	380,124,000
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	97,225,000	88,158,000	97,225,000
Commercial Paper, Fair value	38,061,000	27,363,000	38,061,000
Corporate notes and bonds, Fair Value	46,686,000	57,417,000	46,686,000
Foreign Currency Contract, Asset, Fair Value Disclosure	689,000	143,000	689,000
Total assets at fair value	182,661,000	173,081,000	182,661,000
Liabilities:
Contingent earn-out	0	0	0
Foreign Currency Contracts, Liability, Fair Value Disclosure	(350,000)	(1,192,000)	(350,000)
Total liabilities at fair value	(350,000)	(1,192,000)	(350,000)
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	(1,450,000)	(4,000,000)	(1,450,000)
Foreign Currency Contracts, Liability, Fair Value Disclosure	0	0	0
Total liabilities at fair value	(1,450,000)	(4,000,000)	(1,450,000)
Corporate Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Amortized Cost Basis		57,563,000
Commercial Paper [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Amortized Cost Basis		$ 27,194,000