Commitments and Contingencies (Narrative) (Details) ft² in Thousands	12 Months Ended
	Sep. 30, 2019 USD ($) ft²	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)
Commitments And Contingencies [Line Items]
Lease expense, net of sublease income	$ 39,300,000	$ 36,900,000	$ 35,800,000
Boston lease, square feet | ft²	250,000
Boston lease year one payments	$ 11,000,000.0
Boston lease per square foot annual increase	1
Boston lease annual increase	$ 300,000
Boston lease building operating cost percent	63.00%
Boston lease building operating cost amount estimate year one	$ 7,100,000
Boston lease leasehold improvement allowance	25,000,000
Boston lease leasehold improvement allowance per square foot	100
Letters of credit and bank guarantees outstanding	15,100,000	15,500,000
Bank guarantees outstanding collateralized	1,100,000	$ 1,100,000
Pending or Threatened Litigation [Member]
Commitments And Contingencies [Line Items]
Loss contingency accrual	500,000
Minimum [Member]
Commitments And Contingencies [Line Items]
Loss Contingency, Estimate of Possible Loss	500,000
Maximum [Member]
Commitments And Contingencies [Line Items]
Loss Contingency, Estimate of Possible Loss	1,600,000
Paid in 2017 [Member] | Korea
Commitments And Contingencies [Line Items]
Income Tax Examination, Estimate of Possible Loss	12,000,000
Additional Exposure [Member] | Korea
Commitments And Contingencies [Line Items]
Income Tax Examination, Estimate of Possible Loss	$ 13,000,000