Fair Value Measures - Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands		12 Months Ended
		Sep. 30, 2019	Sep. 30, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt, Fair Value		$ 2,143,400	$ 2,423,600
Long-term debt, face value		2,137,000	2,437,000
Fair Value, Measurements, Recurring
Assets:
Money market funds		217,861	200,004
Bank Time Deposits, Fair Value Disclosure		115,913	88,158
Commercial Paper, Fair value		77,494	27,363
Financial Instruments, Owned, Corporate Debt, at Fair Value		37,566	57,417
Foreign Currency Contract, Asset, Fair Value Disclosure		597	143
Total assets at fair value		449,431	373,085
Foreign Currency Contracts, Liability, Fair Value Disclosure	[1]	(327)	(1,192)
Liabilities:
Contingent earn-out		(2,925)	(4,000)
Total liabilities at fair value		(3,252)	(5,192)
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Assets:
Money market funds		217,861	200,004
Bank Time Deposits, Fair Value Disclosure			0
Commercial Paper, Fair value		0	0
Financial Instruments, Owned, Corporate Debt, at Fair Value			0
Foreign Currency Contract, Asset, Fair Value Disclosure			0
Total assets at fair value		217,861	200,004
Foreign Currency Contracts, Liability, Fair Value Disclosure			0
Liabilities:
Contingent earn-out			0
Total liabilities at fair value		0	0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Assets:
Money market funds		0	0
Bank Time Deposits, Fair Value Disclosure		115,913	88,158
Commercial Paper, Fair value		77,494	27,363
Financial Instruments, Owned, Corporate Debt, at Fair Value		37,566	57,417
Foreign Currency Contract, Asset, Fair Value Disclosure		597	143
Total assets at fair value		231,570	173,081
Foreign Currency Contracts, Liability, Fair Value Disclosure		(327)	(1,192)
Liabilities:
Contingent earn-out			0
Total liabilities at fair value		(327)	(1,192)
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Assets:
Money market funds		0	0
Bank Time Deposits, Fair Value Disclosure		0	0
Commercial Paper, Fair value		0	0
Financial Instruments, Owned, Corporate Debt, at Fair Value		0	0
Foreign Currency Contract, Asset, Fair Value Disclosure		0	0
Total assets at fair value		0	0
Foreign Currency Contracts, Liability, Fair Value Disclosure			0
Liabilities:
Contingent earn-out		(2,925)	(4,000)
Total liabilities at fair value		(2,925)	(4,000)
Commercial Paper [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Amortized Cost Basis		77	27
Corporate Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Amortized Cost Basis		$ 38	$ 58
Commercial paper, corporate notes and bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale Securities, Weighted Average Maturity		6 months 12 days	7 months 11 days

[1] 0.53 years and 0.61 years as of September 30, 2019 and September 30, 2018, respectively.