Income Taxes - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended			65 Months Ended
	Sep. 30, 2017	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018
Income Tax Contingency [Line Items]
Foreign tax rate and other foreign related tax items		$ (1,341)	$ (10,695)	$ (15,768)
Valuation Allowance, Deferred Tax Asset, Increase (Decrease), Amount		$ (120,100)
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent		21.00%	24.50%	35.00%
Stock Repurchased During Period, Value	$ 99,100	$ 126,900	$ 136,100	$ 99,100	$ 1,070,000
Document Period End Date		Sep. 30, 2019
(Benefit) provision for income taxes		$ 88,594	62,320	(23,671)
Undistributed earnings of foreign subsidiaries		243,300
Deferred tax assets valuation allowance		303,378	183,295		183,295
Unrecognized tax benefits potential to favorably impact effective tax rate		$ 85,600
Significant change in unrecognized tax benefits is not expected, period		12 months
Unrecognized tax benefits, interest and penalties accrued		$ 12,700	10,800		10,800
Unrecognized Tax Benefits, Income Tax Penalties and Interest Expense		1,900	1,300	$ 2,000
Investment Tax Credit Carryforward [Member]
Income Tax Contingency [Line Items]
Net operating loss carryforwards		14,300	14,700		14,700
United States
Income Tax Contingency [Line Items]
Deferred tax assets valuation allowance		269,600	142,800		142,800
Net operating loss carryforwards		551,100	692,900		692,900
United States | Research and Development
Income Tax Contingency [Line Items]
Net operating loss carryforwards		27,700	30,200		30,200
State
Income Tax Contingency [Line Items]
Net operating loss carryforwards		194,600	259,100		259,100
State | Research and Development
Income Tax Contingency [Line Items]
Net operating loss carryforwards		3,900	5,300		5,300
Foreign Tax Authority [Member]
Income Tax Contingency [Line Items]
Deferred tax assets valuation allowance		33,800	40,500		40,500
Net operating loss carryforwards		$ 191,700	$ 164,900		$ 164,900