Income Taxes - Aggregate Changes in Balance of Gross Unrecognized Tax Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Income Tax Disclosure [Abstract]
Unrecognized Tax Benefits, Income Tax Penalties and Interest Expense	$ 1,900	$ 1,300	$ 2,000
Reconciliation of Unrecognized Tax Benefits [Roll Forward]
Balance, beginning of year	29,456	33,245
Increases for tax positions taken during the current period	60,225	1,709
Increases for interest and penalty charges	0	1,590
Unrecognized Tax Benefits, Decrease Resulting from Prior Period Tax Positions	0	(2,281)
Decreases for tax settlements and lapse in statutes	(2,291)	(724)
Balance, end of year	85,587	29,456	$ 33,245
Unrecognized Tax Benefit, Increase Resulting from Acquisitions	$ (1,803)	$ (4,083)