Income Taxes - Federal Rate Reconciliation (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Rate reconciliation
Tax at federal statutory rate									$ 85,800	$ 48,800	$ 52,000
State income tax, net of federal benefit									8,000	8,400	7,000
Foreign residual income									45,600	39,500	35,800
Valuation allowance									12,400	(15,900)	(26,500)
Audit settlement									10,400		(4,600)
Foreign tax rate differential									8,800	3,200	(3,100)
Change in uncertain tax positions									8,500	(8,300)	5,600
Nondeductible costs									6,000	15,800	7,600
Income tax credits and incentives									(51,300)	(47,800)	(44,700)
Tax rate changes									(26,800)	(500)	(1,900)
Return to provision									(9,500)	5,100	(200)
Exclusion of tax on non-controlling interests									(6,100)	(3,400)	(5,300)
Tax exempt income									(5,400)	(5,100)	(3,900)
Other items, net									2,600	6,000	(4,300)
Total income tax expense	$ 46,100	$ (17,800)	$ 35,100	$ 25,600	$ 15,300	$ (7,200)	$ 21,700	$ 15,900	$ 89,011	$ 45,753	$ 13,498
Major elements contributing to the difference between the U.S. federal statutory rate of 21% for fiscal year 2020 and 24.5% for fiscal years ended 2019 and 2018 and the effective tax rate
Tax at federal statutory rate (as a percent)									21.00%	21.00%	21.00%
State income tax, net of federal benefit (as a percent)									2.00%	3.60%	2.80%
Foreign residual income (as a percent)									11.10%	17.00%	14.50%
Valuation allowance (as a percent)									3.00%	(6.90%)	(10.70%)
Audit settlement (as a percent)									2.50%		(1.90%)
Foreign tax rate differential (as a percent)									2.10%	1.40%	(1.30%)
Change in uncertain tax positions (as a percent)									2.10%	(3.60%)	2.30%
Nondeductible costs (as a percent)									1.50%	6.80%	3.10%
Income tax credits and incentive (as a percent)									(12.50%)	(20.60%)	(18.10%)
Tax rate changes (as a percent)									(6.50%)	(0.20%)	(0.80%)
Return to provision (as a Percent)									(2.30%)	2.20%	(0.10%)
Exclusion of tax on non-controlling interests (as a percent)									(1.50%)	(1.50%)	(2.10%)
Tax exempt income (as a percent)									(1.30%)	(2.20%)	(1.60%)
Other items, net (as a percent)									0.60%	2.70%	(1.70%)
Total income tax expense (as a percent)		25.00%	19.00%						21.80%	19.70%	5.40%