Income Taxes - Schedule Of Deferred Tax Assets And Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Deferred tax assets:
Net operating loss carryforwards	$ 61,495	$ 26,462
Foreign tax credits	8,074	0
Capitalized research and development	30,109	34,560
Pension benefits	14,370	14,838
Prepaid expenses	13,579	41,739
Deferred revenue	6,021	9,899
Stock-based compensation	13,630	12,306
Other reserves not currently deductible	15,130	20,986
Amortization of intangible assets	162,426	168,376
Research and development and other tax credits	70,695	49,995
Lease liabilities	52,224	0
Fixed assets	47,457	45,450
Capital loss carryforward	35,851	31,248
Deferred interest	0	10,864
Other	1,849	1,623
Gross deferred tax assets	532,910	468,346
Valuation allowance	(205,423)	(177,663)	$ (141,950)	$ (279,683)
Total deferred tax assets	327,487	290,683
Deferred tax liabilities:
Acquired intangible assets not deductible	(65,894)	(42,554)
Lease assets	(35,885)	0
Pension prepayments	(1,155)	(2,532)
Deferred revenue	(594)	(19,312)
Depreciation	(7,481)	0
Unbilled accounts receivable	(12,699)	(31,005)
Deferred income	(5,821)	(19,040)
Prepaid commissions	(17,124)	(17,423)
Other	(2,302)	(1,866)
Total deferred tax liabilities	(148,955)	(133,732)
Net deferred tax assets	$ 178,532	$ 156,951